Improve enqueue_fill_buffer documentation (Closes #206 on github)

doc/runtime_memory.rst

@@ -98,6 +98,12 @@ Buffer
 
     :arg mem: the on device :class:`Buffer`
     :arg pattern: a buffer object (likely a :class:`numpy.ndarray`, eg. `np.uint32(0)`)
+        The memory associated with *pattern* can be reused or freed once the function
+        completes.
+    :arg size: The size in bytes of the region to be filled. Must be a multiple of the
+        size of the pattern.
+    :arg offset: The location in bytes of the region being filled in *mem*.
+        Must be a multiple of the size of the pattern.
 
     Fills a buffer with the provided pattern