fetch-pack: update the documentation for "<refs>..." arguments

When we started allowing an exact object name to be fetched from the
command line, we forgot to update the documentation.

Signed-off-by: Gabriel Souza Franco <gabrielfrancosouza@gmail.com>
--
 Documentation/git-fetch-pack.txt | 4 ++++
 1 file changed, 4 insertions(+)
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fetch-pack.txt

@@ -104,6 +104,10 @@ be in a separate packet, and the list must end with a flush packet.
 	The remote heads to update from. This is relative to
 	$GIT_DIR (e.g. "HEAD", "refs/heads/master").  When
 	unspecified, update from all heads the remote side has.
++
+If the remote has enabled the options `uploadpack.allowTipSHA1InWant` or
+`uploadpack.allowReachableSHA1InWant`, they may alternatively be 40-hex
+sha1s present on the remote.
 
 SEE ALSO
 --------