doc: update applehttp documentation

Integrate more comments from Stefano.

doc/protocols.texi

@@ -25,8 +25,9 @@ Read Apple HTTP Live Streaming compliant segmented stream as
 a uniform one. The M3U8 playlists describing the segments can be
 remote HTTP resources or local files, accessed using the standard
 file protocol.
-HTTP is default, specific protocol can be declared using the "+"
-specifier.
+HTTP is default, specific protocol can be declared by specifying
+"+@var{proto}" after the applehttp URI scheme name, where @var{proto}
+is either "file" or "http".
 
 @example
 applehttp://host/path/to/remote/resource.m3u8
@@ -34,7 +35,6 @@ applehttp+http://host/path/to/remote/resource.m3u8
 applehttp+file://path/to/local/resource.m3u8
 @end example
 
-
 @section concat
 
 Physical concatenation protocol.