ty.rs: document/cleanup required_region_bounds a bit
ty.rs: document/cleanup required_region_bounds a bit

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