Clarify location of server hooks and link documentation

As mentioned in
https://gitlab.com/gitlab-org/gitaly/-/issues/1226#note_202581431, I was
confused by the new location of global server hooks after reading the
comments in these scripts.

This change makes it clear that the default location of the custom
server hooks hasn't actually moved. Only the specific gitlab-shell hook
has been moved to the Gitaly directory.

hooks/post-receive

 #!/bin/sh
-echo "The gitlab-shell hooks have been migrated to Gitaly, see https://gitlab.com/gitlab-org/gitaly/issues/1226"
+echo "This gitlab-shell hook has been migrated to Gitaly. See https://gitlab.com/gitlab-org/gitaly/issues/1226."
+echo
+echo "Note that global server hooks still reside in a subdirectory here by default."
+echo "For example, pre-receive.d/ holds custom pre-receive hooks."
+echo "See https://docs.gitlab.com/ee/administration/server_hooks.html#create-a-global-server-hook-for-all-repositories for more information."
 exit 1
 

hooks/pre-receive

 #!/bin/sh
-echo "The gitlab-shell hooks have been migrated to Gitaly, see https://gitlab.com/gitlab-org/gitaly/issues/1226"
+echo "This gitlab-shell hook has been migrated to Gitaly. See https://gitlab.com/gitlab-org/gitaly/issues/1226."
+echo
+echo "Note that global server hooks still reside in a subdirectory here by default."
+echo "For example, pre-receive.d/ holds custom pre-receive hooks."
+echo "See https://docs.gitlab.com/ee/administration/server_hooks.html#create-a-global-server-hook-for-all-repositories for more information."
 exit 1
 

hooks/update

 #!/bin/sh
-echo "The gitlab-shell hooks have been migrated to Gitaly, see https://gitlab.com/gitlab-org/gitaly/issues/1226"
+echo "This gitlab-shell hook has been migrated to Gitaly. See https://gitlab.com/gitlab-org/gitaly/issues/1226."
+echo
+echo "Note that global server hooks still reside in a subdirectory here by default."
+echo "For example, pre-receive.d/ holds custom pre-receive hooks."
+echo "See https://docs.gitlab.com/ee/administration/server_hooks.html#create-a-global-server-hook-for-all-repositories for more information."
 exit 1