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>

Documentation/git-interpret-trailers.adoc

@@ -115,6 +115,16 @@ 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`::