GH-121462: pathlib docs: improve table of corresponding os/os.path functions (#121465)

Re-order table of corresponding functions with the following priorities:

1. Pure functionality is at the top
2. `os.path` functions are shown before `os` functions
3. Similar functionality is kept together
4. Functionality follows docs order where possible

Add a few missed correspondences:

- `os.path.isjunction` and `Path.is_junction`
- `os.path.ismount` and `Path.is_mount`
- `os.lstat()` and `Path.lstat()`
- `os.lchmod()` and `Path.lchmod()`

Also add footnotes describing a few differences.