Small fixes to ccall documentation.

1. In the "Non-constant Function Specifications" section of the ccall documentation, it had :lib instead of "lib".

2. I explicitly mention the mapping of "void" to "Void". Its implicitly in the docs as there are examples in the page that use it for the return type, but its nice to have it written there.

3. I explicitly mention that you should use Ptr{Uint8} rather than String, and fix a typo in the sentence that follows about char**.