scripts/setlocalversion: add more comments to -dirty flag detection

This script stumbled on the read-only source tree over again:

 - a2bb90a0 ("kbuild: fix delay in setlocalversion on readonly
   source")

 - cdf2bc63 ("scripts/setlocalversion on write-protected source
   tree")

 - 8ef14c2c ("Revert "scripts/setlocalversion: git: Make -dirty
   check more robust"")

 - ff64dd48 ("scripts/setlocalversion: Improve -dirty check with
   git-status --no-optional-locks")

Add comments to clarify that this script should never ever try to write
to the source tree.

'git describe --dirty' might look as a simple solution for appending
the -dirty string, but we cannot use it because it creates the
.git/index.lock file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>