(stable) Partially revert D14511 to fix "INLINE COMMENTS" in mail

Summary:
Ref T9845. In Differential, this is not a remarkup block -- it's a mail section. addTextSection() has special magic behavior when handed a prebuilt section since D9375.

Swapping to addRemarkupSection() causes the error in T9845 and renders nothing in the comment section.

Even if it were a block of text, it would not be appropriate to add it as remarkup. This would incorrectly render comments in files like __init__.py, which are common on Python (the filename would render as "__init__.py"). Okay that's a bad example since it works fine but, uh, a file named T123 would be no good or whatever.

I'll realign T9845 to clean this up and fix it more durably.

Test Plan: Sent myself some mail with inline comments, saw them in the mail.

Reviewers: joshuaspence, chad

Reviewed By: chad

Maniphest Tasks: T9845

Differential Revision: https://secure.phabricator.com/D14589