Add documentation for the syntax for writing (Word-style) comments in markdown

[marviro]

It is possible to write comments in markdown which are then treated by the docx writer as Word comments. The syntax is as follows:

I want [I left a comment.]{.comment-start id="0"
author="Jesse Rosenthal" date="2016-05-09T16:13:00Z"}some text to have a
comment []{.comment-end id="0"}on it.

The pandoc reader parses these comments and in the AST we'll get something like:

[Para [Str "I",Space,Str "want",Space,Span ("0",["comment-start"],[("author", "Jesse Rosenthal"),("date", "2016-05-09T16:13:00Z")]) [Str "I",Space,Str "left",Space,Str "a",Space,Str "comment."],Str "some",Space,Str "text",Space,Str "to",Space,Str "have",Space,Str "a",SoftBreak,Str "comment",Space,Span ("0",["comment-end"],[]) [],Str "on",Space,Str "it."]]

This feature (cf #2994) is not documented in the pandoc manual.

I think it would be useful to add it.

For example, in the part of the manual that talks about pandoc markdown? At the end of the "Inline formatting" section? (

pandoc/MANUAL.txt

Line 5009 in 719f31b

)

I can make a pull request if you agree with this idea @jgm