client: Add detailed docstrings to PdfRestClient public methods

- Enhanced docstrings for all methods in `PdfRestClient` with Google-style
  formatting, detailing parameters, return types, and exceptions for
  improved clarity and usability.
- Included comprehensive descriptions for method functionality to assist
  developers in understanding each operation and its use cases.
- Improved documentation of common arguments like `file`, `timeout`, and
  `extra_*` fields for consistency across methods.

Assisted-by: Codex

Author: datalogics-kam