doc: interpret-trailers: document comment line treatment

Comment lines have always been ignored but this is not documented.

The primary motivation here is to reasonably complete in the
documentation of how trailers are parsed; this is after all the only
documentation page that documents this format. However, and going beyond
that point, we could imagine that someone would want to use this format
outside a commit (or tag) message context, like say in Git notes.

On the other hand, it seems far-fetched that someone would be caught
off guard by this considering that comment characters/strings are not
likely to be alphanumeric,[1] which would mean that these comment lines
would be treated as non-trailer lines if they were *not* detected and
removed as comment lines.

† 1: A notable exception is that Jujutsu VCS uses `JJ:` as
     the comment string

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.adoc

@@ -117,6 +117,16 @@ key: This is a very long value, with spaces and
   newlines in it.
 ------------------------------------------------
 
+OTHER RULES
+-----------
+
+What was covered in the previous section are the rules that are relevant
+for regular use. The following points are included for completeness.
+
+This command ignores comment lines (see `core.commentString` in
+linkgit:git-config[1]). This is for use with the `prepare-commit-msg`
+and `commit-msg` hooks.
+
 OPTIONS
 -------
 `--in-place`::