[ahbaqdadi]

The Dataset reference does not clearly distinguish methods that mutate the receiver from methods that return a new dataset. This makes filter() easy to read as an in-place operation, as reported in #348.

Add a mutation/return-value table, clarify the individual transformation, filtering, sorting, de-duplication and composition sections, and show retaining the filtered dataset separately. Also correct the reverseApply() signature and its Reversible link.

Fixes #348.

Validation:

• 125 behavior checks across Labeled and Unlabeled datasets confirm the documented return identities and immediate mutation behavior.
• Full PHPUnit suite: 2,141 tests, 51,310 assertions; 38 skips for optional extensions.
• PHPStan and the 728-file code style check pass.
• MkDocs builds successfully; the new table and corrected anchor are verified in the rendered HTML. An existing unrelated missing cross-entropy.md link warning remains.
• git diff --check passes.

Documentation-only; runtime code and tests are unchanged. AI-assisted with Codex; the behavior statements were checked against the current implementations and runtime fixtures.