Clarify Fread() and Fwrite() semantics

These have claimed to be fread() / fwrite() "clones" but never *really*
were, and have been more or less consciously directed toward read()/write()
style semantics long ago, eg 3208913bdb
and 289ba88b30 talk about this. Changing
this long-standing behavior now would be little gain for some unnecessary
pain to callers, might just as well simply document the behavior and
be done with it.

rpmio/rpmio.h

@@ -35,12 +35,12 @@ typedef const struct FDIO_s * FDIO_t;
 const char * Fstrerror(FD_t fd);
 
 /** \ingroup rpmio
- * fread(3) clone.
+ * Like fread(3) but with read(3)-style return values.
  */
 ssize_t Fread(void * buf, size_t size, size_t nmemb, FD_t fd);
 
 /** \ingroup rpmio
- * fwrite(3) clone.
+ * Like fwrite(3) but with write(3)-style return values.
  */
 ssize_t Fwrite(const void * buf, size_t size, size_t nmemb, FD_t fd);