[Devel] documentation fixes

[Vincent Caron]

I found those notes on a forgotten draft under my keyboard... :) 

- Tutorial Step 2 - 2. a. Extracting the glyph image: 

  error = FT_Load_Glyph( face, glyph, FT_LOAD_NORMAL ); 

should read as : 

  error = FT_Load_Glyph( face, glyph_index, FT_LOAD_NORMAL ); 


- FT_Get_Kerning() 

'kern_mode' input doc : doesn't document the behaviour for fixed size
fonts (nor in FT_Kerning_Mode), however it is expected that only
ft_kerning_default will be supported (?) 

'akerning' output doc : a bit misleading at first sight, the vector unit
for scalable fonts actually depends on kern_mode. I guess it should
redirect the user to either kern_mode or FT_Kerning_Mode. 

BTW, do fixed size fonts happen to define kerning pairs ? (for my own
culture, thanks :))