First draft documenting Debug stability.
First draft documenting Debug stability.

Debug implementations of std types aren't stable, and neither are
derived Debug implementations for any types, including user-defined
types. This commit adds a section to the Debug documentatio noting this
stability status.

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