[ink_storage] Add missing docs for ink_storage_derive proc. macros #637 (#711)

* [ink_storage] Add missing docs for ink_storage_derive. (paritytech/ink#637).

* Update crates/storage/derive/src/lib.rs

Co-authored-by: Hero Bird <robbepop@web.de>

* Update crates/storage/derive/src/lib.rs

Co-authored-by: Hero Bird <robbepop@web.de>

* Update crates/storage/derive/src/lib.rs

Co-authored-by: Hero Bird <robbepop@web.de>

* Update crates/storage/derive/src/lib.rs

Co-authored-by: Hero Bird <robbepop@web.de>

* Fix fmt.

Co-authored-by: Hero Bird <robbepop@web.de>
18 jobs for master in 7 minutes and 44 seconds (queued for 2 seconds)