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>