Mention that you have to pre-multiply the alpha in al_map_rgba[_f]
Fix building of the PDF docs
Mention that PhysFS and memfile addons do not support text mode
Syntax highlight the C source examples and prototypes in the documentation.

Interestingly, while doing this I hit a bug with the syntax highlighter in
Pandoc:

 ~~~~c
 #include <stdio.h>
 ~~~~

Would render mis-highlighted and missing the 'e' in include. Adding a leading
space to the code does not trigger the bug.