doc: interpret-trailers: document comment line treatment

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

This is mostly for completeness since this is unlikely to catch anyone
by surprise. But we really ought to be reasonably complete here since
it’s the only documentation page that documents trailers.

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

Author: LemmingAvalanche

Documentation/git-interpret-trailers.adoc

@@ -103,6 +103,10 @@ of the message in turn is either (i) at the end of the input, or (ii)
 the last non-whitespace lines before a line that starts with `---`
 (followed by a space or the end of the line).
 
+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.
+
 When reading trailers, there can be no whitespace before or inside the
 _<key>_, but any number of regular space and tab characters are allowed
 between the _<key>_ and the separator. There can be whitespaces before,