Add src/librustdoc as an alias for src/tools/rustdoc
Add src/librustdoc as an alias for src/tools/rustdoc

No one actually works with src/tools/librustdoc, it's almost empty.

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