Expands release note instructions. (#16696)

* Expands release note instructions.

* Updates pluralization for consistency.

* Removes explicit reference to release note block.

* Removes unnecessary reason for past-tense.

* Adds message about newlines being stripped.

* Adds other criteria for release notes.

.github/PULL_REQUEST_TEMPLATE.md

@@ -18,11 +18,35 @@ If this pull request addresses a Help Wanted ticket, please link the relevant Gi
 
 Otherwise, link the JIRA ticket.
 -->
+
 #### Release Note
 <!--
-If no release notes are required, just write "NONE" in the release-note block below.
-If yes, a release note is required:
-Enter your past-tense release note in the block below.
+Add a release note for each of the following conditions:
+
+* Config changes (additions, deletions, updates)
+* API additions—new endpoint, new response fields, or newly accepted request parameters
+* Database changes (any)
+* Websocket additions or changes
+* Anything noteworthy to a Mattermost instance administrator (err on the side of over-communicating)
+* New features and improvements, including behavioural changes, UI changes and CLI changes
+* Bug fixes and fixes of previous known issues
+* Deprecation warnings, breaking changes, or compatibility notes
+
+If no release notes are required write NONE. Use past-tense. Newlines are stripped.
+
+Examples:
+
+```release-note
+Added new API endpoints POST /api/v4/foo, GET api/v4/foo, and GET api/v4/foo/:foo_id.
+```
+
+```release-note
+Added a new config setting ServiceSettings.FooBar. Added a new column Foo to the Users table.
+```
+
+```release-note
+NONE
+```
 -->
 ```release-note
 ```