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GH-31507: [Python] Address docstrings in Streams and File Access (Stream Classes) (#33698)


### Rationale for this change
Ensure docstrings for [Streams and File Access](https://arrow.apache.org/docs/python/api/files.html) - Stream Classes - have an Examples section.

### What changes are included in this PR?
Docstrings are added to listed Stream Classes:
- OSFile
- PythonFile
- BufferReader
- BufferOutputStream
- FixedSizeBufferWriter
- MemoryMappedFile
- CompressedInputStream
- CompressedOutputStream

### Are these changes tested?
Yes, locally with `pytest --doctest-cython --disable-warnings pyarrow` and on the CI with `Python / AMD64 Conda Python 3.9 Sphinx & Numpydoc` build.

### Are there any user-facing changes?
No.
* Closes: #31507

Lead-authored-by: Alenka Frim <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Alenka Frim <frim.alenka@gmail.com>