Document the fact that Rouge only recognises lower-case language identifiers

[nisbet-hubbard]

Why:

GitHub accepts both ```Diff and ```diff for the purposes of syntax highlighting, but GitHub Pages will only highlight a ```diff code block.

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

Documented this caveat.

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 a 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.

[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
get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10
pages/setting-up-a-github-pages-site-with-jekyll/about-github-pages-and-jekyll.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 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]

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

[felicitymay]

Hi @nisbet-hubbard

Thanks for raising this pull request. You learn something new every day. I hadn't realized that you could use anything other than lower-case 😄

I've suggested some changes to the wording and added a link to the supported language list for Rouge. I'd normally leave my comments open for a few days to give you an opportunity to come back to me with your thoughts on the suggestions. However, I'm going to be out of the office over the holiday period, so I'll apply my changes and merge this so the information is published without a long delay.

[github-actions]

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