B2 SDK docs: convert embedded TODO directives into tracked documentation issues

[ffumero2003]

Description

Several Sphinx pages contain embedded todo directives for missing explanations, raw-simulator coverage, list consistency, exception documentation, progress documentation, and an advanced-API design concern. doc/source/conf.py only includes them when B2_SPHINX_TODO is enabled. In the normal published build they are invisible, and they carry no owner, priority, acceptance criteria, or discussion history.

Steps to reproduce

1. Search doc/source/index.rst, advanced.rst, and doc/source/api for `todo::` or `TODO::`.
2. Build the documentation without B2_SPHINX_TODO enabled.
3. Observe that the pending work is absent from the normal output and is not linked to tracked issues.

Expected behavior

Review each TODO and classify it as obsolete, documentation work, API design work, or intentionally deferred. Remove obsolete entries, create focused issues for actionable work, and replace any retained source TODO with a link to its issue and a reason it remains useful in the source.

Actual behavior

Maintenance work is hidden inside source files and can remain unnoticed indefinitely. Some TODOs describe user-facing documentation gaps, while another describes a possible API change; keeping them together without tracking also obscures different owners and compatibility risks.

Environment

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