docs: comment on EDXAPP_PRIVATE_REQUIREMENTS (#6991)

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

playbooks/roles/edxapp/defaults/main.yml

@@ -540,6 +540,9 @@ EDXAPP_EXTRA_REQUIREMENTS: []
 # Example:
 # EDXAPP_PRIVATE_REQUIREMENTS:
 #   - name: git+https://git.myproject.org/MyProject#egg=MyProject
+# Note: This list contains edx.org specific dependencies, even though this is
+#  a public repo. The plan is to phase this out along with the rest of this
+#  repo as part of the DEPR https://github.com/openedx/public-engineering/issues/51.
 EDXAPP_PRIVATE_REQUIREMENTS:
     # For Harvard courses:
     - name: xblock-problem-builder==5.1.3