Improve the fsaa setting description (minetest#10279)

See minetest#8459.

builtin/settingtypes.txt

@@ -513,8 +513,13 @@ texture_clean_transparent (Clean transparent textures) bool false
 #    texture autoscaling.
 texture_min_size (Minimum texture size) int 64
 
-#    Experimental option, might cause visible spaces between blocks
-#    when set to higher number than 0.
+#    Use multi-sample antialiasing (MSAA) to smooth out block edges.
+#    This algorithm smooths out the 3D viewport while keeping the image sharp,
+#    but it doesn't affect the insides of textures
+#    (which is especially noticeable with transparent textures).
+#    This option is experimental and might cause visible spaces between blocks
+#    when set above 0.
+#    A restart is required after changing this option.
 fsaa (FSAA) enum 0 0,1,2,4,8,16
 
 #    Undersampling is similar to using a lower screen resolution, but it applies