Mark images in Service Alerts as final (google#651)

Remove the experimental tag from Alert.image, Alert.image_alternative_text
and the TranslatedImage message, in both the .proto and reference.md.

Author: gcamp

gtfs-realtime/proto/gtfs-realtime.proto

@@ -708,12 +708,10 @@ message Alert {
 
   // TranslatedImage to be displayed along the alert text. Used to explain visually the alert effect of a detour, station closure, etc. The image must enhance the understanding of the alert. Any essential information communicated within the image must also be contained in the alert text.
   // The following types of images are discouraged : image containing mainly text, marketing or branded images that add no additional information. 
-  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
   optional TranslatedImage image = 15;
 
   // Text describing the appearance of the linked image in the `image` field (e.g., in case the image can't be displayed
   // or the user can't see the image for accessibility reasons). See the HTML spec for alt image text - https://html.spec.whatwg.org/#alt.
-  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
   optional TranslatedString image_alternative_text = 16;
 
 
@@ -1055,7 +1053,6 @@ message TranslatedString {
 //    translation, the first matching translation is picked.
 // 3. If some translation has an unspecified language code, that translation is
 //    picked.
-// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
 message TranslatedImage {
   message LocalizedImage {
     // String containing an URL linking to an image

gtfs-realtime/spec/en/reference.md

@@ -376,8 +376,8 @@ An alert, indicating some sort of incident in the public transit network.
 | **tts_header_text** | [TranslatedString](#message-translatedstring) | Optional | One | Text containing the alert's header to be used for text-to-speech implementations. This field is the text-to-speech version of header_text. It should contain the same information as header_text but formatted such that it can read as text-to-speech (for example, abbreviations removed, numbers spelled out, etc.) |
 | **tts_description_text** | [TranslatedString](#message-translatedstring) | Optional | One | Text containing a description for the alert to be used for text-to-speech implementations. This field is the text-to-speech version of description_text. It should contain the same information as description_text but formatted such that it can be read as text-to-speech (for example, abbreviations removed, numbers spelled out, etc.) |
 | **severity_level** | [SeverityLevel](#enum-severitylevel) | Optional | One | Severity of the alert. |
-| **image** | [TranslatedImage](#message-translatedimage) | Optional | One | TranslatedImage to be displayed along the alert text. Used to explain visually the alert effect of a detour, station closure, etc. The image should enhance the understanding of the alert and must not be the only location of essential information. The following types of images are discouraged : image containing mainly text, marketing or branded images that add no additional information. <br><br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future. |
-| **image_alternative_text** | [TranslatedString](#message-translatedstring) | Optional | One | Text describing the appearance of the linked image in the `image` field (e.g., in case the image can't be displayed or the user can't see the image for accessibility reasons). See the HTML [spec for alt image text](https://html.spec.whatwg.org/#alt). <br><br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future. |
+| **image** | [TranslatedImage](#message-translatedimage) | Optional | One | TranslatedImage to be displayed along the alert text. Used to explain visually the alert effect of a detour, station closure, etc. The image should enhance the understanding of the alert and must not be the only location of essential information. The following types of images are discouraged : image containing mainly text, marketing or branded images that add no additional information. |
+| **image_alternative_text** | [TranslatedString](#message-translatedstring) | Optional | One | Text describing the appearance of the linked image in the `image` field (e.g., in case the image can't be displayed or the user can't see the image for accessibility reasons). See the HTML [spec for alt image text](https://html.spec.whatwg.org/#alt). |
 
 
 ## _enum_ Cause
@@ -589,8 +589,6 @@ A localized string mapped to a language.
 
 An internationalized message containing per-language versions of an image. One of the images from a message will be picked up. The resolution proceeds as follows: If the UI language matches the language code of a translation, the first matching translation is picked. If a default UI language (e.g., English) matches the language code of a translation, the first matching translation is picked. If some translation has an unspecified language code, that translation is picked.
 
-**Caution:** this message is still **experimental**, and subject to change. It may be formally adopted in the future.
-
 **Fields**
 
 | _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |