B2 SDK docs: clean up stale statements, invalid terminology, and recurring prose errors #591

Description

The overview and Advanced Guide contain stale or unclear statements and recurring language errors, including overalapping, manullay, "Streams that contains", "If all candidates fails", and the outdated statement that the SDK "has been developed for 3 years" before version 1.0. These reduce clarity in already complex sections.

Steps to reproduce

Search doc/source/index.rst and doc/source/advanced.rst for:
overalapping, manullay, "Streams that contains", and "If all candidates fails".

Expected behavior

Published documentation should use current, precise terminology and clear grammar, especially in explanations of advanced transfer behavior.

Actual behavior

Multiple errors and time-relative statements remain in published pages.

Environment

• Python version: Not applicable
• b2sdk version: master at f3ab7b71
• OS: Not applicable