document some limitations on repo topics

[hopkinsth]

Why:

There are some limitations on repository topics, and without more documentation, the only way to know about them is trial-and-error.

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

Added a few bullet points describing the list of limitations on topics that I have found.

Check off the following:

• [x] I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• [x] For content changes, I have completed the self-review checklist.

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[github-actions[bot]]

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
repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/classifying-your-repository-with-topics.md	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae

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

[cmwilson21]

@hopkinsth Thanks so much for opening a PR! I'll get this triaged for review :zap:

[hopkinsth]

@janiceilene Thanks for the review! I have moved the new text to the end of the "About topics" section, and please let me know if this needs any more tweaks.

[janiceilene]

This looks great @hopkinsth! Thank you so much ⚡ We're frozen for a bit while folks are on holiday, but I'll get this added to the list of things to merge once we're thawed ❄️

[github-actions[bot]]

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