api-credentials.txt: add "see also" section

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: moy

Documentation/technical/api-credentials.txt

@@ -289,3 +289,10 @@ request.
 If a helper receives any other operation, it should silently ignore the
 request. This leaves room for future operations to be added (older
 helpers will just ignore the new requests).
+
+See also
+--------
+
+linkgit:../gitcredentials[7]
+
+linkgit:../git-config[5] (See configuration variables `credential.*`)