Help documentation does not contain information about "Quote selected in Reply" feature

[rtanglao]

• From a 14 year old bug, bug 572146 Help documentation does not contain information about "Quote selected in Reply" feature
• See also Bug 23394 Quote just the selected portion of a message during Reply

  
  The feature ( only selected part of the original message is quoted in the reply) is present, but users are not informed about it even via help files.
  An accidental selection in the original message will produce results different from the expected by a user and my lead to an incomplete reply being sent.

Reproducible: Always

Steps to Reproduce:

1. Try to find the description of the feature in "Help".

3. Actual Results:
   No information found.

Expected Results:
Information about the feature.

- [ ] figure which articles to change