avformat: Document urls a bit

Spell-checked-by: Moritz Barsnick <barsnick@gmx.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avformat.h

@@ -78,6 +78,18 @@
  * if its AVClass is non-NULL, and the protocols layer. See the discussion on
  * nesting in @ref avoptions documentation to learn how to access those.
  *
+ * @section urls
+ * URL strings in libavformat are made of a scheme/protocol, a ':', and a
+ * scheme specific string. URLs without a scheme and ':' used for local files
+ * are supported but deprecated. "file:" should be used for local files.
+ *
+ * It is important that the scheme string is not taken from untrusted
+ * sources without checks.
+ *
+ * Note that some schemes/protocols are quite powerful, allowing access to
+ * both local and remote files, parts of them, concatenations of them, local
+ * audio and video devices and so on.
+ *
  * @defgroup lavf_decoding Demuxing
  * @{
  * Demuxers read a media file and split it into chunks of data (@em packets). A