docs/manual: document usage of BR2_TOOLCHAIN_HAS_SYNC_x

This commit updates the documentation to detail when and how to use
the BR2_TOOLCHAIN_HAS_SYNC_x dependency. Note that we chose to not add
a comment about this dependency, because it is mainly tied to
architecture capabilities (except in very specific cases, which would
be way too complicated to explain in a Config.in comment).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>