Documentation: mention 'git stash pop --index' option explicitly

Documentation: mention 'git stash pop --index' option explicitly

'git stash pop' supports the '--index' option since its initial
implementation (bd56ff54, git-stash: add new 'pop' subcommand,
2008-02-22), but its documentation does not mention it explicitly.
Moreover, both the usage shown by 'git stash -h' and the synopsis
section in the man page imply that 'git stash pop' does not have an
'--index' option.

First, this patch corrects the usage and the synopsis section.

Second, the patch moves the description of the '--index' option to the
'git stash pop' section in the documentation, and refers to it from
the 'git stash apply' section.  This way it follows the intentions of
commit d1836637 (Documentation: teach stash/pop workflow instead of
stash/apply, 2009-05-28), as all 'git stash pop'-related documentation
will be in one place without references to 'git stash apply'.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>