docs/manual: suggest 'YES' as the value for LIBFOO_GIT_SUBMODULES

docs/manual: suggest 'YES' as the value for LIBFOO_GIT_SUBMODULES

The manual mentions LIBFOO_GIT_SUBMODULES but does not tell what value
it should have. The implementation only checks whether the variable is
non-empty, but we should suggest a specific value to avoid people
wondering what they have to set, and to try setting weird values.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Ricardo Herrero <ricardo.herrero@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>