doc: clarify how exit status of post-checkout hook is used

Because the hook runs after the main checkout operation finishes, it
cannot affect what branch will be the current branch, what paths are
updated in the working tree, etc., which was described as "cannot
affect the outcome of 'checkout'".

However, the exit status of the hook is used as the exit status of
the 'checkout' command and is observable by anybody who spawned the
'checkout', which was missing from the documentation.  Fix this.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

Documentation/githooks.txt

@@ -193,7 +193,9 @@ worktree.  The hook is given three parameters: the ref of the previous HEAD,
 the ref of the new HEAD (which may or may not have changed), and a flag
 indicating whether the checkout was a branch checkout (changing branches,
 flag=1) or a file checkout (retrieving a file from the index, flag=0).
-This hook cannot affect the outcome of `git switch` or `git checkout`.
+This hook cannot affect the outcome of `git switch` or `git checkout`,
+other than that the hook's exit status becomes the exit status of
+these two commands.
 
 It is also run after linkgit:git-clone[1], unless the `--no-checkout` (`-n`) option is
 used. The first parameter given to the hook is the null-ref, the second the