LGA-2677: Updated documentation regarding BACKEND_BASE_URI in .example.local.py

[BenMillar-MOJ]

What does this pull request do?

Adds comments in the example.local.py file, documenting a unique interaction where changing an environment variable will not have the desired effect unless the Zone Profile dictionary is also updated.

Any other changes that would benefit highlighting?

Intentionally left blank.

Checklist

• [x] Provided JIRA ticket number in the title, e.g. "LGA-152: Sample title"

[sonarcloud[bot]]

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