docs: comment on EDXAPP_PRIVATE_REQUIREMENTS

[robrap]

Add a note to clarify that EDXAPP_PRIVATE_REQUIREMENTS contains edx.org specific details, which is not ideal. The plan is to ultimately retire this repo according to the DEPR: openedx/public-engineering#51

Configuration Pull Request

Make sure that the following steps are done before merging:

• A SRE team member has approved the PR if it is code shared across multiple services and you don't own all of the services.
• Are you adding any new default values that need to be overridden when this change goes live? If so:
  • Update the appropriate internal repo (be sure to update for all our environments)
  • If you are updating a secure value rather than an internal one, file a SRE ticket with details.
  • Add an entry to the CHANGELOG.
• If you are making a complicated change, have you performed the proper testing specified on the Ops Ansible Testing Checklist? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).
• Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?

[timmc-edx]

One typo, but looks good.