gh-149763: Improve availablity docs in select.rst

[sobolevn]

• Issue: Improve "Availability" docs in select.rst #149763

[hugovk]

Doc/tools/extensions/availability.py defines the allowed platforms.

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Whilst we're editing this, please could you also update the headers to use sentence case?

"Edge and level trigger polling (epoll) objects" and so on.

https://devguide.python.org/documentation/style-guide/#capitalization

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Also in this table: https://docs.python.org/3/library/select.html#edge-and-level-trigger-polling-epoll-objects

• Change "assoc. fd" to "associated fd" or "associated file descriptor"
• "polling on a fd" -> "polling on an fd" or "polling on a file descriptor"
• Some of the Meaning column entries end with a full stop, some don't. Let's add them to all.

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Elsewhere:

• "process id" -> "process ID"
• "Register a fd descriptor" -> "Register a file descriptor"
• "Filter specific" -> "Filter-specific"
• "User defined value" -> "User-defined value"

[sobolevn]

@hugovk thanks a lot for an amazing review 🤝

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #32675761 | 📁 Comparing 2d5721d against main (6304eb1)

  🔍 Preview build  

2 files changed

± library/select.html
± whatsnew/changelog.html

[hugovk]

Thanks! Please could you update the other headers:

/dev/poll Polling Objects
Polling Objects
Kqueue Objects
Kevent Objects