docs/139603: [patch] Clarify subpixel smoothing for fonts in Handbook

[Norbert Papke]

Although some smoothing is performed even with the option disabled, enabling 
the option does improve the visual quality of the rendered font.  IMHO the 
difference is quite apparent.

There are some note about this in the API documentation:

http://freetype.sourceforge.net/freetype2/docs/reference/ft2-
lcd_filtering.html

Cheers,

-- Norbert Papke.