Clarify GITHUB_SHA for push event

[dannycjones]

Why:

Simple clarification on the value of GITHUB_SHA for push event. It's fairly obvious but with this change, there's no room for ambiguity.

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

Just clarifying the content of GITHUB_SHA. First sentence under GITHUB_SHA is newly added.

Webhook event payload	Activity types	GITHUB_SHA	GITHUB_REF
push	Not applicable	Commit pushed to the ref. When you delete a branch, the SHA in the workflow run (and its associated refs) reverts to the default branch of the repository.	Updated ref

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[github-actions]

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Content directory changes

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Source	Preview	Production	What Changed
actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows.md	fpt ghec ghes@ 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.14 3.13 3.12 3.11 3.10

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

[nguyenalex836]

@dannycjones Thanks so much for opening a PR! I'll get this triaged for review ✨

[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀

[nguyenalex836]

@dannycjones Thank you so much for your patience while our SME team reviewed! They found a small to the wording would offer increased clarity, which I have added as a suggestion 💛

Once that suggestion has been applied, we will be happy to get this merged!

[dannycjones]

Thanks @nguyenalex836, I incorporated the feedback and also merged from main.

[nguyenalex836]

@dannycjones Thank you for applying that suggestion! I'll update the branch and get this merged once tests are passing 🍏

[github-actions]

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