Propagate RUSTDOCFLAGS in the environment when documenting
Propagate RUSTDOCFLAGS in the environment when documenting

Previously, RUSTDOCFLAGS would get overriden when bootstrap set
`RUSTDOCFLAGS` itself. Propagate the flag manually, using the same logic
as `RUSTFLAGS`.

This also extracts the logic into a helper function to make sure it's
the same.

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