doc: log, gitk: document accepted line-log diff formats

doc: log, gitk: document accepted line-log diff formats

Currently the line-log functionality (git log -L) only supports
displaying patch output (`-p | --patch`, its default behavior) and suppressing it
(`-s | --no-patch`). A check was added in the code to that effect in 5314efaea
(line-log: detect unsupported formats, 2019-03-10) but the documentation was not
updated.

Explicitly mention that `-L` implies `-p`, that patch output can be
suppressed using `-s`, and that all other diff formats are not allowed.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>