doc/codecs: Add missing documentation for nointra

Add missing documentation for nointra.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

doc/codecs.texi

@@ -963,6 +963,9 @@ Discard all bidirectional frames.
 @item nokey
 Discard all frames excepts keyframes.
 
+@item nointra
+Discard all frames except I frames.
+
 @item all
 Discard all frames.
 @end table