Explanation of the different "multi_compile" directives?

Where can I find what those multi_compile directives are? I have been looking and looking but can't find any info on these specific multi_compile directives:

multi_compile_builtin
multi_compile_fwdbase
multi_compile_fwdadd

The #pragma documentation doesn't list them, and I didn't find any other pages which talks about #pragma multi_compile directives.

If I missed it in the docs then shame on me

Thanks in advance guys...
Stephane

 

I find this:http://docs.unity3d.ru/Components/SL-ShaderPrograms.html