Add common synonyms

[henrebotha]

Why:

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adding synonyms for "alerts" to make them easier to find by searching.

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• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Source	Review	Production	What Changed
get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax.md	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Sharra-writes]

@henrebotha Thanks for the PR! I'll accept this one based mostly on the fact that we actually tend to refer to them internally as "callouts," and I keep forgetting what term I'm supposed to be searching for when I'm trying to look up something about them.

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