Rewrite doxy for av_strtod().

Originally committed as revision 19268 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/eval.h

@@ -74,9 +74,22 @@ AVEvalExpr * ff_parse(const char *s, const char * const *const_name,
 double ff_parse_eval(AVEvalExpr * e, const double *const_value, void *opaque);
 void ff_eval_free(AVEvalExpr * e);
 
-/** strtod() function extended with 'k', 'M', 'G', 'ki', 'Mi', 'Gi' and 'B'
- * postfixes.  This allows using f.e. kB, MiB, G and B as a postfix. This
- * function assumes that the unit of numbers is bits not bytes.
+/**
+ * Parses the string in numstr and returns its value as a double. If
+ * the string is empty, contains only whitespaces, or does not contain
+ * an initial substring that has the expected syntax for a
+ * floating-point number, no conversion is performed. In this case,
+ * returns a value of zero and the value returned in tail is the value
+ * of numstr.
+ *
+ * @param numstr a string representing a number, may contain one of
+ * the International System number postfixes, for example 'K', 'M',
+ * 'G'. If 'i' is appended after the postfix, powers of 2 are used
+ * instead of powers of 10. The 'B' postfix multiplies the value for
+ * 8, and can be appended after another postfix or used alone. This
+ * allows using for example 'KB', 'MiB', 'G' and 'B' as postfix.
+ * @param tail if non-NULL puts here the pointer to the char next
+ * after the last parsed character
  */
 double av_strtod(const char *numstr, char **tail);