Prescribe something about type parameter documentation

[djc]

Here's an example I came across today, from an underdocumented part of nom:

http://rust.unhandledexpression.com/nom/struct.MapConsumer.html

Which, to wit, has <'a, C: 'a, R, S, T, E, M, F> as type parameters, and fails to explain even a single one of them.

For another example, tokio-proto has this for ClientProxy:

https://tokio-rs.github.io/tokio-proto/tokio_proto/util/client_proxy/struct.ClientProxy.html

The use of R, S, and E are not that obvious until you look at the associated types, but looking at the top-level documentation it's pretty unclear what these types signify. It would be great if these guidelines could mandate (a) more readable type parameter names, or, failing that, (b) clear documentation on the meaning of the type parameters for particular types.