cmd/go: add documentation for go.work.sum

[samherrmann]

With the release of Go 1.18, we now have workspaces. From my findings, workspaces appear to be documented in the following locations:

• https://go.dev/doc/gopath_code#Workspaces
• https://go.dev/doc/tutorial/workspaces
• https://github.com/golang/go/blob/go1.18/src/cmd/go/internal/workcmd/work.go

None of these locations however make mention of the go.work.sum file. The go.work.sum file was however described in the proposal.

This issue is to bring the description of the go.work.sum file from the proposal into the official documentation.

[mknyszek]

CC @matloob