type_decoder: move Python doc strings to method implementations

type_decoder: move Python doc strings to method implementations

Move Python doc strings to the location where methods and classes get
implemented. This improves awareness during maintenance, and allows for
longer text phrases without obfuscating the registration code path.

This commit does not alter the content of existing doc strings, only
moves their location. And keeps related items together when long decls
span multiple text lines (function name and arguments list, args and
kw args), to improve/keep readability.