lavu/dict.h: apply various fixes/extensions to doxy

In particular, document AV_DICT_MATCH_CASE, AV_DICT_IGNORE_SUFFIX, and
extend documentation for av_dict_get().

libavutil/dict.h

@@ -64,12 +64,13 @@
  *
  */
 
-#define AV_DICT_MATCH_CASE      1
-#define AV_DICT_IGNORE_SUFFIX   2
+#define AV_DICT_MATCH_CASE      1   /**< Only get an entry with exact-case key match. Only relevant in av_dict_get(). */
+#define AV_DICT_IGNORE_SUFFIX   2   /**< Return first entry in a dictionary whose first part corresponds to the search key,
+                                         ignoring the suffix of the found key string. Only relevant in av_dict_get(). */
 #define AV_DICT_DONT_STRDUP_KEY 4   /**< Take ownership of a key that's been
-                                         allocated with av_malloc() and children. */
+                                         allocated with av_malloc() or another memory allocation function. */
 #define AV_DICT_DONT_STRDUP_VAL 8   /**< Take ownership of a value that's been
-                                         allocated with av_malloc() and chilren. */
+                                         allocated with av_malloc() or another memory allocation function. */
 #define AV_DICT_DONT_OVERWRITE 16   ///< Don't overwrite existing entries.
 #define AV_DICT_APPEND         32   /**< If the entry already exists, append to it.  Note that no
                                       delimiter is added, the strings are simply concatenated. */
@@ -84,8 +85,12 @@ typedef struct AVDictionary AVDictionary;
 /**
  * Get a dictionary entry with matching key.
  *
+ * To iterate through all the dictionary entries, you can set the matching key
+ * to the null string "" and set the AV_DICT_IGNORE_SUFFIX flag.
+ *
  * @param prev Set to the previous matching element to find the next.
  *             If set to NULL the first matching element is returned.
+ * @param key matching key
  * @param flags Allows case as well as suffix-insensitive comparisons.
  * @return Found entry or NULL, changing key or value leads to undefined behavior.
  */