Update the docs with explicit unittest to clarify usage of flag attribute everyone

[imohib]

For more context, please refer to #561. Basically, current version of the docs incorrectly suggests that setting waffle flag attribute everyone overrides all the other setting unless set to Unknown (i.e. None).

Proposed changes in this PR:

1. Fixes the docs re: incorrect usage of the waffle flag attribute everyone.
2. Adds testcase WaffleFlagEveryoneSettingTests to prevent breaking changes and makes the behaviour explicit
3. Fixes the warning - "django-waffle/docs/starting/installation.rst:54: WARNING: Explicit markup ends without a blank line; unexpected unindent."

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

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  Hits              ?     2371           
  Misses            ?      103           
  Partials          ?        0           

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