Strongly discourage --update-head-ok in fetch-options documentation.

"Use it with care" is a wrong wording to say "this is purely internal
and you are supposed to know what you are doing if you use this".
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/fetch-options.txt

@@ -36,6 +36,8 @@
 -u, \--update-head-ok::
 	By default `git-fetch` refuses to update the head which
 	corresponds to the current branch.  This flag disables the
-	check.  Note that fetching into the current branch will not
-	update the index and working directory, so use it with care.
+	check.  This is purely for the internal use for `git-pull`
+	to communicate with `git-fetch`, and unless you are
+	implementing your own Porcelain you are not supposed to
+	use it.