diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 3888d9f..f844213 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -6154,11 +6154,34 @@
 
 		Example: >
 		  set encoding=utf-8
-		  set gfn=Ricty_Diminished:h12:cSHIFTJIS
+		  set gfn=Ricty_Diminished:h12
 		  set rop=type:directx
 <
-		If select a raster font (Courier, Terminal or FixedSys) to
-		'guifont', it fallbacks to be drawn by GDI automatically.
+		If select a raster font (Courier, Terminal or FixedSys which
+		have ".fon" extension in file name) to 'guifont', it will be
+		drawn by GDI as a fallback.  This fallback will cause
+		significant slow down on drawing.
+
+		NOTE: It is known that some fonts and options combination
+		causes trouble on drawing glyphs.
+
+		  - 'rendmode:5' and 'renmode:6' will not work with some
+		    special made fonts (True-Type fonts which includes only
+		    bitmap glyphs).
+		  - 'taamode:3' will not work with some vector fonts.
+
+		NOTE: With this option, you can display colored emoji
+		(emoticon) in Windows 8.1 or later.  To display colored emoji,
+		there are some conditions which you should notice.
+
+		  - If your font includes non-colored emoji already, it will
+		    be used.
+		  - If your font doesn't have emoji, the system chooses an
+		    alternative symbol font.  On Windows 10, "Segoe UI Emoji"
+		    will be used.
+		  - When this alternative font didn't have fixed width glyph,
+		    emoji might be rendered beyond the bounding box of drawing
+		    cell.
 
 	Other render types are currently not supported.