Mention that `instance` tag is normalized

[yzhan289]

What does this PR do?

The http_check's instance tag is being normalized, and therefore all instance:test-instance is converted into instance:test_instance, even though the public docs mention that - are valid for tags.

This PR adds a note in the name config option to mention that this is happening, since this is where instance tag pulls its data.

Motivation

Support case

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached

[codecov[bot]]

Codecov Report

Merging #12719 (b57c474) into master (9715860) will increase coverage by 1.50%. The diff coverage is n/a.

Flag	Coverage Δ
datadog_checks_base	90.13% <ø> (+9.41%)	:arrow_up:
http_check	95.38% <ø> (+2.08%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

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