[issue39090] Document various options for getting the absolute path from pathlib.Path objects

[John Hennig]

John Hennig <jhen at gmx.net> added the comment:

@Floris:
> Not mentioning Path.resolve()'s behavior w.r.t. non-existing files since
that's documented in resolve() itself.

I don't see it mentioned in the documentation of `resolve()`, or anywhere else in the docs, that on Windows (but not on other platforms) `resolve()` does *not* resolve a relative path to an absolute path if the file does not exist. As opposed to `absolute()`, which works as expected on any platform.

Linux:
```
Python 3.6.9 (default, Oct  8 2020, 12:12:24)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pathlib import Path
>>> file = Path('new.txt')
>>> file.exists()
False
>>> file.resolve()
PosixPath('/home/user/new.txt')
```

Windows:
```
Python 3.9.2 (tags/v3.9.2:1a79785, Feb 19 2021, 13:44:55) [MSC v.1928 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from pathlib import Path
>>> file = Path('new.txt')
>>> file.exists()
False
>>> file.resolve()
WindowsPath('new.txt')
>>> file.absolute()
WindowsPath('d:/home/new.txt')
>>> file.touch()
>>> file.resolve()
WindowsPath('D:/home/new.txt')
>>> file.unlink()
>>> file.resolve()
WindowsPath('new.txt')
>>>
```

----------
nosy: +John-Hennig

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<https://bugs.python.org/issue39090>
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