Added information about group assignment of whitelisted/blacklisted d…

[martinbrose]

…omains

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:
Addressing issue pi-hole/pi-hole#4581 by adding additional information about group assignment of whitelisted/blacklisted domains.

How does this PR accomplish the above?:
Added two sentences to guides / misc / whitelist-blacklist.md.

What documentation changes (if any) are needed to support this PR?:
No additional changes necessary. This PR is a documentation change.
No additional comments within the code necessary. Hence, I left the related checkbox empty above.

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[martinbrose]

Thanks for your review @yubiuser.
Agreed and changed.

I probably should also slightly change the wording for the related PR pi-hole/web#2186.