docs: correct S7CommPlus support guidance - #839

Summary

• document the legacy V1 SessionKey path instead of claiming a PUT/GET fallback
• clarify the synchronous and asynchronous authentication boundaries
• replace the nonexistent S7CommPlus read_tag example with the current read_symbolic API
• distinguish classic S7 symbol/layout limitations from S7CommPlus browsing capabilities
• correct the remaining browse() docstrings and runtime warning so they no longer advertise nonexistent APIs or fallback behavior

Related to #710, #836, and #837.

Validation

• uv run --frozen sphinx-build -E -N -b html doc/ doc/_build -W
• uv run --frozen pytest (1821 passed, 78 skipped)
• uv run --frozen pre-commit run --all-files
• uv build --no-sources