The mutable-matrix row and column operations return the matrix but document no output

This issue was triaged from [`bugs/dan/1-doc-rowMult`](https://github.com/Macaulay2/M2/blob/388c1ff0ce30d83751dea7bc7eac77fdc1305dd7/bugs/dan/1-doc-rowMult), one of the 857 files removed from the pre-GitHub `bugs/` tree by [`d2c8d27826`](https://github.com/Macaulay2/M2/commit/d2c8d27826) and catalogued in [#36](https://github.com/Macaulay2/M2/issues/36). **The commentary below was written by Claude (Claude Opus 5, via Claude Code)**, not by @d-torrance, whose account posted it -- please weigh it accordingly.

### The original file, verbatim

```text
rowMult returns a value not mentioned in its doc
the other col and row functions might also
```

### Where it stands today

True as reported, and the file's suspicion that *"the other col and row functions might also"* is
confirmed for the whole family.

`rowMult(m, i, a)` returns the `MutableMatrix` it just modified. `help rowMult` documents Usage, Inputs
and Consequences, and has **no Outputs section** — so the return value appears only incidentally, as
`o3` in the example.

### The whole family behaves the same way

`rowMult`, `columnMult`, `rowAdd`, `columnAdd`, `rowSwap`, `columnSwap` **and** `rowPermute` all return
the `MutableMatrix` rather than `null`, and none of those seven documentation nodes has an Outputs
section.

### Small and closable

Two acceptable fixes: document the return value in an Outputs section, or state in Consequences that the
matrix is returned for convenience. Either settles all seven nodes, since they share the pattern.

_{`open` · disposition `issue` · source of truth: [`bug-triage/catalog.tsv`](https://github.com/d-torrance/M2/blob/bug-triage/bug-triage/catalog.tsv)}

Start by inspecting the documentation nodes for rowMult, columnMult, rowAdd, columnAdd, rowSwap, columnSwap, and rowPermute, using their existing help entries as the entry point. Document that each operation returns the modified MutableMatrix, either in an Outputs section or in Consequences, and verify the updated help output for all seven functions.

Written by the indexing model from the issue text.