Merge 5.2 into 5.x (#3340)

Author: mongodb-php-bot

Diff for: docs/compatibility.txt

@@ -34,3 +34,9 @@ PHP Driver Compatibility
 
 To use {+odm-long+} v5.2 or later, you must install v1.21 of the
 {+php-library+} and {+php-extension+}.
+
+.. important:: {+php-extension+} v2.0 Compatibility
+
+   If you upgrade the {+php-extension+} to v2.0, you must also upgrade
+   {+odm-long+} to v5.2.1. {+odm-long+} v5.2.1 still supports v1.x
+   versions of the extension.