Clarify WAL retention policy documentation

[puneetdixit200]

Summary

• update the FAQ so it no longer implies that WAL files can have an independent retention policy
• clarify that wal_retention_policy = main follows the main retention_policy setting

Closes #1183.

Validation

• git diff --check
• PYTHONPATH=. uvx --from sphinx --with sphinx-github-style --with sphinxcontrib-apidoc --with pydata-sphinx-theme --with myst_parser --with python-dateutil --with psycopg2-binary sphinx-build -b html docs /tmp/barman-docs-html -q --keep-going exited 0; the log still contains existing unrelated documentation warnings, with none for the touched files
• uvx tox -e docs -- html could not complete locally because the tox env tried to build psycopg2 and pg_config is not installed

Assisted-by: OpenAI GPT-5