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

### Code of Conduct

- [X] I have read and agree to the GitHub Docs project's [Code of Conduct](https://github.com/github/docs/blob/main/CODE_OF_CONDUCT.md)

### 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.` would be case sensitive.

Here's a subset of the workflow they had:

```yaml
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++
```