`derive` explanation for PartialOrd should match that for Ord
`derive` explanation for PartialOrd should match that for Ord

I think these just got out of sync, but both use a lexicographic
ordering.

Relevant commits in the history of these explanations:
* 8b81f76 on 2015-06-30
* e22770b on 2016-02-09

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