docs/fb: Document current named modes

KMS supports a number of named modes already, but it's never been
documented anywhere, let's fix that.
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v9-1-24b168e5bcd5@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>

Documentation/fb/modedb.rst

@@ -26,6 +26,11 @@ Valid mode specifiers (mode_option argument)::
 with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
 Things between square brackets are optional.
 
+Valid names are::
+
+  - NSTC: 480i output, with the CCIR System-M TV mode and NTSC color encoding
+  - PAL: 576i output, with the CCIR System-B TV mode and PAL color encoding
+
 If 'M' is specified in the mode_option argument (after <yres> and before
 <bpp> and <refresh>, if specified) the timings will be calculated using
 VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.