docs: clarify table write API docstrings

[BharatDeva]

Rationale for this change

Issue #1056 tracks missing or incomplete docstrings in PyIceberg public APIs. While reviewing the table write helpers, I noticed a few small gaps in the Table API docs:

• append described the input dataframe as overwriting the table, although the method appends rows.
• add_files documented only file_paths, even though the public method also accepts snapshot properties, duplicate-file checking, and branch selection.
• update_spec did not have a public docstring, unlike the surrounding table update helpers.

What changes are included in this PR?

This PR makes a small documentation-only update to pyiceberg/table/__init__.py:

• clarifies the append argument description
• adds missing add_files argument descriptions
• adds a short update_spec docstring with its case_sensitive argument

Are these changes tested?

Yes. I ran the following local checks:

• uv tool run ruff@0.14.3 check --config pyproject.toml pyiceberg/table/__init__.py
  • Result: All checks passed!
• uv tool run pydocstyle@6.3.0 --ignore=D100,D102,D101,D103,D104,D107,D203,D212,D213,D404,D405,D406,D407,D411,D413,D415,D417 pyiceberg/table/__init__.py
  • Result: passed with no output
• git diff --check
  • Result: passed with no whitespace errors

Are there any user-facing changes?

Yes, documentation-only. The public API docs are clearer, but runtime behavior is unchanged.

LLM-generated code disclosure

This documentation update was prepared with assistance from OpenAI Codex and manually reviewed before submission.