submodule foreach: document '$sm_path' instead of '$path'

submodule foreach: document '$sm_path' instead of '$path'

As using a variable '$path' may be harmful to users due to
capitalization issues, see 64394e3ae9 (git-submodule.sh: Don't
use $path variable in eval_gettext string, 2012-04-17). Adjust
the documentation to advocate for using $sm_path,  which contains
the same value. We still make the 'path' variable available and
document it as a deprecated synonym of 'sm_path'.

Discussed-with: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>