ty: add doc comments to `Generics` methods
ty: add doc comments to `Generics` methods

This commit adds doc comments to the `param_at`, `region_param` and
`const_param` methods on the `Generics` struct.

Signed-off-by: David Wood <aa743a0aaec8f7d7a1f01442503957f4d7a2d634@davidtw.co>

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust