Clarify vars and secrets context availability in composite actions - #46249

Why:

Closes: #46232

Inside composite actions, contexts such as secrets and vars are not directly available to the action's steps because variable contexts are filtered and resolved at the calling job level (as noted in the Composite Actions ADR and actions/runner#4311).

While the documentation for the secrets context explicitly notes that it is unavailable in composite actions and must be passed as an input, the documentation for the vars context previously did not mention this, leading users to assume vars could be accessed directly in composite actions.

What's being changed:

• content/actions/reference/workflows-and-actions/contexts.md: Updated the vars context section to clarify that vars is not available directly within composite actions, mirroring the existing clarification in the secrets context section.
• content/actions/tutorials/create-actions/create-a-composite-action.md: Added a note explaining that composite actions do not have direct access to secrets or vars contexts, and that values should be passed explicitly using inputs.
• content/actions/reference/workflows-and-actions/metadata-syntax.md: Clarified under composite action inputs that secrets and vars contexts are not directly available inside composite actions.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• 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.