doc: mention user-configured trailers

In commit 14624506 ("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: NJonathan Tan <jonathantanmy@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.txt

@@ -49,7 +49,8 @@ will be added before the new trailer.
 
 Existing trailers are extracted from the input message by looking for
 a group of one or more lines that (i) are all trailers, or (ii) contains at
-least one Git-generated trailer and consists of at least 25% trailers.
+least one Git-generated or user-configured trailer and consists of at
+least 25% trailers.
 The group must be preceded by one or more empty (or whitespace-only) lines.
 The group must either be at the end of the message or be the last
 non-whitespace lines before a line that starts with '---'. Such three