doc: mention user-configured trailers
In commit
1462450 ("trailer: allow non-trailers in trailer block",
2016-10-21), functionality was added (and tested [1]) to allow
non-trailer lines in trailer blocks, as long as those blocks contain at
least one Git-generated or user-configured trailer, and consists of at
least 25% trailers. The documentation was updated to mention this new
functionality, but did not mention "user-configured trailer".
Further update the documentation to also mention "user-configured
trailer".
[1] "with non-trailer lines mixed with a configured trailer" in
t/t7513-interpret-trailers.sh
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>