man/io_uring_enter.2: note that not all requests support fixed files
@@ -899,7 +899,10 @@ is an index into the files array registered with the io_uring instance (see the
.B IORING_REGISTER_FILES
section of the
.BR io_uring_register (2)
-man page). Available since 5.1.
+man page). Note that this isn't always available for all commands. If used on
+a command that doesn't support fixed files, the SQE will error with
+.B -EBADF.
+Available since 5.1.
.TP
.B IOSQE_IO_DRAIN
When this flag is specified, the SQE will not be started before previously
