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>

Documentation/config/core.txt

@@ -68,6 +68,17 @@ core.fsmonitor::
 	avoiding unnecessary processing of files that have not changed.
 	See the "fsmonitor-watchman" section of linkgit:githooks[5].
 
+core.fsmonitorHookVersion::
+	Sets the version of hook that is to be used when calling fsmonitor.
+	There are currently versions 1 and 2. When this is not set,
+	version 2 will be tried first and if it fails then version 1
+	will be tried. Version 1 uses a timestamp as input to determine
+	which files have changes since that time but some monitors
+	like watchman have race conditions when used with a timestamp.
+	Version 2 uses an opaque string so that the monitor can return
+	something that can be used to determine what files have changed
+	without race conditions.
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time

Documentation/githooks.txt

@@ -490,9 +490,16 @@ fsmonitor-watchman
 ~~~~~~~~~~~~~~~~~~
 
 This hook is invoked when the configuration option `core.fsmonitor` is
-set to `.git/hooks/fsmonitor-watchman`.  It takes two arguments, a version
-(currently 1) and the time in elapsed nanoseconds since midnight,
-January 1, 1970.
+set to `.git/hooks/fsmonitor-watchman` or `.git/hooks/fsmonitor-watchmanv2`
+depending on the version of the hook to use.
+
+Version 1 takes two arguments, a version (1) and the time in elapsed
+nanoseconds since midnight, January 1, 1970.
+
+Version 2 takes two arguments, a version (2) and a token that is used
+for identifying changes since the token. For watchman this would be
+a clock id. This version must output to stdout the new token followed
+by a NUL before the list of files.
 
 The hook should output to stdout the list of all files in the working
 directory that may have changed since the requested time.  The logic