Add documentation for string->Cow conversions
Add documentation for string->Cow conversions

Mostly, it's just to reassure everyone that these functions don't allocate.

Part of #51430

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