gh-129011: Update docs for Raw I/O read, readinto, and write (#135328)

gh-129011: Update docs for Raw I/O read, readinto, and write (#135328)

Update `RawIOBase` and `FileIO` documentation to match implementation
behavior around `.read`, `.readinto`, `.readall` and `.write`.

In particular:

 - They may make more than one system call (PEP-475)
 - Add warnings if `.write()` requires a wrapping retry loop (see: gh-126606)
    - "Raw I/O" `.write`` may not write all bytes
    - `buffering=0` example results in a "Raw I/O"

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>