Update all comments and documentation in Package Manager to refer to "sandboxes" and related terminology

Problem/Motivation

This is the second part of #3474876: Rename classes that reference a "stage" to be clearer and use the term "sandbox" instead. In that issue, we renamed many classes in Package Manager to use the clearer terminology proposed in #3474876-5: Rename classes that reference a "stage" to be clearer and use the term "sandbox" instead. To keep the scope manageable, lest the MR turn into a ceaselessly mutating Tetsuo-like blob monster (see Akira if you haven't already), all documentation and comment updates have been deferred to this issue. It need not block a beta version of Package Manager in core, but it does block a stable release.

To help things along, we might also consider doing things in this issue like removing constructor documentation entirely, since core doc comments don't need them, and in most cases they're pretty useless.