`std::time::Duration`: improve _precision_ of terminology in docs
`std::time::Duration`: improve _precision_ of terminology in docs

Changed wording of docs on `std::time::Duration` for better clarity
w.r.t. the contents of the type and the purpose of its methods.

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