Clarify case-(in)sensitivity for matrix.<keys>

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions
https://docs.github.com/en/actions/learn-github-actions/contexts#matrix-context

What part(s) of the article would you like to see updated?

Add an indication whether or not matrix elements are case sensitive.

Additional information

Some sections indicate if items are case sensitive / case insensitive.

Developers of one project w/ which I have some affiliation were asking whether matrix.<key> would be case sensitive.

Here's a subset of the workflow they had:

jobs:
  linux:
    strategy:
      fail-fast: false
      matrix:
        arch:
          - amd64
        include:
          - arch: amd64
            CC: x86_64-linux-gnu
          - arch: arm64
            CC: aarch64-linux-gnu
    runs-on: ubuntu-22.04
    env:
      AR: ${{ matrix.CC }}-ar
      CHOST: ${{ matrix.cc }}
      CC: ${{ matrix.cc }}-gcc
      CPP: ${{ matrix.cc }}-cpp
      CXX: ${{ matrix.cc }}-g++

[cmwilson21]

@jsoref Thanks for opening an issue and providing the workflow. I'll get this triaged for review 👀