Remove $MEMORY_LIMIT disclaimer

Now that $MEMORY_LIMIT is available during staging, the disclaimer
about it is removed.

[#52785711]
Merge 52785711-memory-limit-docs to master

[Completes #52785711]
Add diagnostics and a test for a case which fails on run.pivotal.io
Merge branch 'master' into close-to-default-memory-bug