Generic documentation TODOs

[Paul Sokolovsky]

• Add placeholder docs for ffi module.
• Add real docs for ffi module.
• Fully document ure module
• Document urandom module - placeholder added
• Update docs for ussl module
• Fully document uctypes module (#14)
• Document abstract stream interface (uio module)
• Document extended return codes (WANT_READ, WANT_WRITE) for non-blocking streams.