doc: log, gitk: move '-L' description to 'line-range-options.txt'

doc: log, gitk: move '-L' description to 'line-range-options.txt'

The description of the '-L' option for `git log` and `gitk` is almost
the same, but is repeated in both 'git-log.txt' and 'gitk.txt' (the
difference being that 'git-log.txt' lists the option with a space
after '-L', while 'gitk.txt' lists it as stuck and notes that `gitk`
only understands the stuck form).

Reduce duplication by creating a new file, 'line-range-options.txt',
and include it in both files.

To simplify the presentation, only list the stuck form for both
commands, and remove the note about `gitk` only understanding the stuck
form.

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