Add consideration for not allowing this before the first read

[zgroza]

No description provided.

[jyasskin]

Just putting this behind clipboard-read isn't equivalent to rejecting before the document first calls read(), since that permission could be remembered from a different visit. You can probably just remove the second sentence, unless there's a use case I'm not seeing for getting the content-id before reading within a given visit?

Suggested change

	- The returned promise should be rejected before the document first successfully calls `read()` or `readText()` within a
	session. In particular, a browser that implements persistent clipboard permissions might just put this behind
	`clipboard-read`.
	- The returned promise should be rejected before the document first successfully calls `read()` or `readText()` within a
	session.

[zgroza]

So, this isn't equivalent, yes - but the entire idea is to forbid applications that have no business using clipboard from using this. If a site has full access to the clipboard anyway because of the persistent permission, it would make sense not to bar it from using this API.