Add note about reserved tags in Multi Alert

[khy1226]

What does this PR do? What is the motivation?

• Based on ESC comment (https://datadoghq.atlassian.net/browse/MNTS-93910?focusedCommentId=3447609), clarify that reserved tag keys are available in Multi Alert regardless of the configured group-by dimensions.
• Add a note to the documentation to clarify this behavior.

Merge readiness

• Ready for merge

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AI assistance

Additional notes

[OliviaShoup]

hey, thanks for the PR! left a comment but let me know if you have questions!

[OliviaShoup]

putting these two notes together to make it a bit easier to scan. also changing the link format to our style guide convention. i can't comment on that part of the file, but this needs to go at the very end of the link list on line 772:

Suggested change

	**Note**: When you use the `group_by` field in aggregation, additional tags and alerts from the monitor may be inherited automatically. This means that any alerts or configurations set on the monitored endpoint could be applied to each group resulting from the aggregation.
	**Note**: [Reserved tag keys](https://docs.datadoghq.com/getting_started/tagging/) will exist independently regardless of the group-by dimension
	**Note**: When you use the `group_by` field in aggregation, additional tags and alerts from the monitor may be inherited automatically, so any alerts or configurations set on the monitored endpoint could apply to each group resulting from the aggregation. [Reserved tag keys][21] are exceptions. They're available in every alert regardless of the selected group-by dimension.