Rollup merge of #42920 - behnam:cmp, r=BurntSushi
Rollup merge of #42920 - behnam:cmp, r=BurntSushi

[libcore/cmp] Expand Ord/PartialOrd Derivable doc for enum types

Expand Derivable docblock section for `Ord` and `PartialOrd` to cover
`enum` types, in addition to the existing language explaining it for
`struct` types.

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