(Release: TBH) feat(docs): update to nrql alert conditions docs

[akane0915]

Please follow conventional commit standards in your commit messages and pull request title.

Give us some context

What problems does this PR solve?

https://new-relic.atlassian.net/browse/NR-270157

A few new features are almost ready to be released in alerts:

1. Users can create "stand alone loss of signal" NQRL alert conditions i.e. a condition can have LoS only and no critical/warning numeric threshold configured
2. Users have a new option in the LoS settings to prevent an incident from opening when a signal is expected to be lost
3. Users can create a host not reporting NRQL condition now (in place of legacy infra HNR conditions)

Important: These new features have not been released to production yet (they're almost ready), so we should not merge this PR until someone from the DCON team has notified you.

Add any context that will help us review your changes such as testing notes, links to related docs, screenshots, etc.

https://newrelic.atlassian.net/wiki/spaces/AppInt/pages/3389816880/Loss+of+Signal+Conditions+w+Expected+Termination+-+MMF+Technical+Implementation#Goals

Note: I added a few // TODO comments in the PR to add/update screenshots. I'm not sure how to do this, so maybe a docs hero can help me?

If your issue relates to an existing GitHub issue, please link to it. N/A

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