APIchanges: mention avcodec_alloc_context()/2/3

There was no minor bump for making avcodec_alloc_context3() public and
deprecating the other two, so I'm using the first next lavc bump.

doc/APIchanges

@@ -163,6 +163,8 @@ API changes, most recent first:
 
 2011-07-10 - 0b950fe - lavc 53.6.0
   Add avcodec_open2(), deprecate avcodec_open().
+  Add avcodec_alloc_context3. Deprecate avcodec_alloc_context() and
+  avcodec_alloc_context2().
 
 2011-06-23 - 67e9ae1 - lavu 51.8.0 - attributes.h
   Add av_printf_format().