doc: log, gitk: line-log arguments must exist in starting revision

doc: log, gitk: line-log arguments must exist in starting revision

The line number, regex or offset parameters <start> and <end> in
`git log -L <start>,<end>:<file>`, or the function name regex in
`git log -L :<funcname>:<file>` must exist in the starting
revision, or else the command exits with a fatal error.

This is not obvious in the documentation, so add a note to that
effect.

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