More on how INPUT_<VARIABLE_NAME> works or doesn't

[jsoref]

This is a follow-up to #7693

Why:

actions/runner#665 (comment)

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

Removing prose that hints that a composite action would get INPUT_ prefixed environment variables.

Instead, just point to the top of the doc where I previously added text that seems to describe what's happening.

Before

After

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Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/creating-actions/metadata-syntax-for-github-actions.md	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae

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

[cmwilson21]

👋 @jsoref Thanks for opening a PR! I'll get this triaged for review ⚡

[lucascosti]

Great change, thanks @jsoref!

[github-actions]

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