Add more documentation for entities, fixes #1038 (#1041)
Add more documentation for entities, fixes #1038 (#1041)

Also fixes broken links to point to docs.rs; see
https://github.com/CraneStation/cranelift/pull/1041#issuecomment-532356880,
there doesn't currently seem to be a way to link to a type in a
dependent crate.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne