Suffix in pathlib is not behaving like a file extension

Bug report

Bug description:

According to the docs here a suffix is defined as:

The file extension of the final component, if any

But pathlib doesn't behave as expected, illustrating that on Python 3.12.4:

>>> Path("Mr. Smith resume for review").suffix
'. Smith resume for review'

This is particularly problematic for methods like with_suffix. According to the docs here that should:

Return a new path with the suffix changed. If the original path doesn’t have a suffix, the new suffix is appended instead. If the suffix is an empty string, the original suffix is removed

But as established above, that results in:

>>> Path("Mr. Smith resume for review").with_suffix(".pdf")
PosixPath('Mr.pdf')

I've characterized that as a bug as that is not working as described, but could also be a matter of documentation improvement.

I see a few ways:

1. Implement add_suffix, or an argument to with_suffix, but I know that has been discussed before and was ultimately decided against. Also this is a matter of how suffix is defined, not how it's processed.
2. Sanity check what a suffix can be, e.g. suffix can't have white space. But ultimately difficult without making assumption.
3. Adjust the documentation: Clarify that suffix is not equal to file extension but something like last segment separated by a dot or similar. Maybe even with a warning that this can have unexpected behavior as showcased above.

CPython versions tested on:

3.12

Operating systems tested on:

Linux, Windows

先重現 Python 3.12 中關於 pathlib 的 PurePath.suffix 和 with_suffix 範例，然後檢查 pathlib 的實作與文件，了解後綴是如何定義的。將觀察到的行為與文件中關於檔案副檔名的措辭進行比較，並判斷該問題是透過行為變更解決，還是透過澄清文件解決。