fsmonitor: add documentation for the fsmonitor extension.

This includes the core.fsmonitor setting, the fsmonitor integration hook,
and the fsmonitor index extension.

Also add documentation for the new fsmonitor options to ls-files and
update-index.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>