docs: Fix docstring typos, add missing docs groups, and document platform purge()

[vdusek]

Documentation-only portion of the v4 major-release cleanup.

• Fix docstring typos and stale claims in Actor: MIIGRATING → MIGRATING, drop the nonexistent call_task content_type argument, drop the "in seconds" note on timeout (the parameter is a timedelta), correct the exit_process default note, and fix the task_id description.
• Fix Configuration docstrings: "prising" → "pricing", and the concatenated web_server_port description.
• Replace mkdocs-style cross-references in events/_types.py with plain backticks.
• Add @docs_group plus a docstring to ApifyRequestQueueMetadata, and @docs_group to ApifyFileSystemStorageClient.
• Document in the v4 upgrading guide that platform storages raise NotImplementedError on purge() (use drop() instead). Local file-system storages are unaffected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.91%. Comparing base (0daca28) to head (653f080).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #982   +/-   ##
=======================================
  Coverage   89.90%   89.91%           
=======================================
  Files          49       49           
  Lines        3091     3094    +3     
=======================================
+ Hits         2779     2782    +3     
  Misses        312      312           

Flag	Coverage Δ
e2e	35.97% <100.00%> (+0.06%)	⬆️
integration	56.91% <100.00%> (+0.04%)	⬆️
unit	78.76% <100.00%> (+0.02%)	⬆️

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