AVoption doxy: clarify a few needs in relation to AVClass less structs.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.h

@@ -44,7 +44,7 @@
  * This section describes how to add AVOptions capabilities to a struct.
  *
  * All AVOptions-related information is stored in an AVClass. Therefore
- * the first member of the struct must be a pointer to an AVClass describing it.
+ * the first member of the struct should be a pointer to an AVClass describing it.
  * The option field of the AVClass must be set to a NULL-terminated static array
  * of AVOptions. Each AVOption must have a non-empty name, a type, a default
  * value and for number-type AVOptions also a range of allowed values. It must
@@ -81,7 +81,7 @@
  * @endcode
  *
  * Next, when allocating your struct, you must ensure that the AVClass pointer
- * is set to the correct value. Then, av_opt_set_defaults() must be called to
+ * is set to the correct value. Then, av_opt_set_defaults() can be called to
  * initialize defaults. After that the struct is ready to be used with the
  * AVOptions API.
  *