Document underscores in numeric literals

src/asm/rgbasm.5

@@ -105,6 +105,8 @@ There are a number of numeric formats.
 .It Gameboy graphics Ta \` Ta 0123
 .El
 .Pp
+Underscores are also accepted in numbers, except at the beginning of decimal ones.
+.Pp
 The "character constant" form yields the value the character maps to in the current charmap.
 For example, by default
 .Pq refer to Xr ascii 7