Improve docstrings of functions for moving to message headers

Improve docstrings of functions for moving to message headers

* lisp/gnus/message.el (message-goto-to, message-goto-from,
message-goto-subject, message-goto-cc, message-goto-bcc,
message-goto-fcc, message-goto-reply-to,
message-goto-newsgroups, message-goto-distribution,
message-goto-followup-to, message-goto-mail-followup-to,
message-goto-keywords, message-goto-summary): Mention that these
will insert an empty header if the header is not found.
(message-position-on-field): Explain that this inserts an empty
header if the header is missing, and that insertion is done
after the headers mentioned in AFTERS.

(cherry picked from commit 9f266e2d7cde41f5872304bae0b6d2415655f1c8)