docs: Set minimal gtags / GNU GLOBAL version to 6.6.5

Kernel build now uses the gtags "-C (--directory)" option, available
since GNU GLOBAL v6.6.5.  Update the documentation accordingly.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Cc: <stable@vger.kernel.org>
Link: https://lists.gnu.org/archive/html/info-global/2020-09/msg00000.html


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>