Improve the cleanness of the rustdoc output

[Veetaha]

Here is how the rustdoc output looks like for the example from the Documenting section. It isn't pretty, but we may probably do better. For example, leading __ in generic parameters and lifetimes may be avoided if we document that some symbol name patterns are reserved for the generated builder signatures to avoid name conflicts, and make the macro smart enough to avoid name conflicts with the symbols in it's scope