doc/protocols: document protocol_whitelist

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/protocols.texi

+@chapter Protocol Options
+@c man begin PROTOCOL OPTIONS
+
+The libavformat library provides some generic global options, which
+can be set on all the protocols. In addition each protocol may support
+so-called private options, which are specific for that component.
+
+The list of supported options follows:
+
+@table @option
+@item protocol_whitelist @var{list} (@emph{input})
+Set a ","-separated list of allowed protocols. "ALL" matches all protocols. Protocols
+prefixed by "-" are disabled.
+All protocols are allowed by default but protocols used by an another
+protocol (nested protocols) are restricted to a per protocol subset.
+@end table
+
+@c man end PROTOCOL OPTIONS
+
 @chapter Protocols
 @c man begin PROTOCOLS