Apply "Clarified the paragraph concerning sane_read"

api.rst

@@ -1187,8 +1187,8 @@ This function is used to read image data from the device represented by
 handle :data:`h`. Argument :data:`buf` is a
 pointer to a memory area that is at least :data:`maxlen`
 bytes long. The number of bytes returned is stored in
-:data:`*len`. A backend must set this to zero when the call
-fails (i.e., when a status other than
+:data:`*len`. A backend must set this to zero when
+a status other than
 :macro:`SANE_STATUS_GOOD` is returned). When the call
 succeeds, the number of bytes returned can be anywhere in the range from
 0 to :data:`maxlen` bytes.