Annotate code references inside inline documentation in Python-SDK

Some of the inline documentation in Python-SDK contains references to classes, methods or fields. The document generation tool (Sphinx) supports annotating such code blocks.

If the inline documentation is annotated properly then the references will be linked (clickable) on the generated site.

Relevant Sphinx documentation:
http://www.sphinx-doc.org/en/stable/markup/inline.html#xref-syntax

Imported from Jira [BEAM-1756](https://issues.apache.org/jira/browse/BEAM-1756). Original Jira may contain additional context.
Reported by: tibor.kiss@gmail.com.

Start by locating the Python-SDK inline documentation that references classes, methods, or fields, then review Sphinx's inline xref syntax documentation. Work through the relevant references and verify the generated site makes them clickable; the task is done when the applicable code references are annotated consistently.

Written by the indexing model from the issue text.