fsmonitor: update documentation for hook version and watchman hooks

fsmonitor: update documentation for hook version and watchman hooks

A new config value for core.fsmonitorHookVersion was added to be able
to force the version of the fsmonitor hook.  Possible values are 1 or 2.
When this is not set the code will use a value of -1 and attempt to use
version 2 of the hook first and if that fails will attempt version 1.

Signed-off-by: Kevin Willford <Kevin.Willford@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>