backport fix doublemapkey (#2480)

* fix double map encoding

* decl_storage doc (#2341)

* doc

* other doc cleanup and fixing links

* one more typo

* decl_storage document hasher choice (#2387)

* doc

* fix

* grammar and line wrapping

* small changes to end

* Update lib.rs

* update after demi's review

* bump runtime

* update doc