rtwn(4): refresh manpage

- Remove 'if_rtwn_load="YES"' line from loader.conf; the module was
renamed in r319733 + it will be loaded automatically as a dependency.
- Move new sentence to new line.
- Add short description for dev.rtwn.%d.rx_buf_size tunable.

share/man/man4/rtwn.4

@@ -18,7 +18,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 24, 2017
+.Dd July 31, 2017
 .Dt RTWN 4
 .Os
 .Sh NAME
@@ -44,7 +44,6 @@ Alternatively, to load the driver as a
 module at boot time, place following lines in
 .Xr loader.conf 5 :
 .Bd -literal -offset indent
-if_rtwn_load="YES"
 if_rtwn_pci_load="YES"
 if_rtwn_usb_load="YES"
 .Ed
@@ -53,7 +52,8 @@ The
 .Nm
 driver provides support for wireless network devices based on
 the Realtek RTL8192C, RTL8188E, RTL8192E, RTL8812A and RTL8821A
-programming APIs.  These APIs are used by a wide variety of chips;
+programming APIs.
+These APIs are used by a wide variety of chips;
 most chips with USB and some with PCI interface are supported.
 .Pp
 To enable use for PCI/PCIe systems, see the rtwn_pci(4) driver;
@@ -175,6 +175,9 @@ algorithm in case if it is not implemented
 Currently selected algorithm is reported via
 .Em Va dev.rtwn.%d.ratectl_selected
 read-only OID.
+.It Va dev.rtwn.%d.rx_buf_size
+(USB only) Controls size of temporary Rx buffer; smaller buffer size
+may increase number of interrupts.
 .El
 .Sh DIAGNOSTICS
 .Bl -diag