Rollup merge of #124904 - RalfJung:reachable, r=tmiasko

reachable computation: extend explanation of what this does, and why

Follow-up to rust-lang/rust#122769. I had the time to think about this some more, in particular in the context of rust-lang/rust#119214, so I felt it was worth extending these comments some more.

I also gave up on the context of "externally reachable" as it is not called that way anywhere else in the compiler.

Cc `@tmiasko` `@saethlin`