Comments refer to docs/storage/nonmoving-gc design document that doesn't exist

Some of the comments in the rts's implementation of the non-moving gc refer to a design document that can be found at docs/storage/nonmoving-gc but afaict that doesn't exist.

Presumably it is the same as this document https://www.well-typed.com/blog/aux/files/nonmoving-gc/design.pdf

I feel like either the link should be updated or the document should be checked in to git.