Document how to declare the typical value of a binary operator method

This issue was triaged from [`bugs/dan/0-typical-values`](https://github.com/Macaulay2/M2/blob/388c1ff0ce30d83751dea7bc7eac77fdc1305dd7/bugs/dan/0-typical-values), 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
document the typical return value construction used here:

X + X := X => (x,y) -> ...
```

### Where it stands today

Undocumented, and the form works — which is the awkward combination, since a user has no way to learn
about a feature that is fully functional.

### What is missing

`X + X := X => (x,y) -> ...` records a typical value: after it, `typicalValues#(symbol +, X, X)` is `X`,
and the documentation machinery uses that when it renders the method. But:

- **"specifying typical values"** shows only the unary form, `prune Matrix := Matrix => f`;
- **"binary methods"** does not mention it;
- **"installing methods"** does not mention it.

So the binary spelling is nowhere in the manual.

### Related work already done

**PR #3102** (merged) used typical values when documenting *assignment* methods, so `AtomicInt += ZZ`
renders with the right type — evidence that the mechanism is actively relied on, and a natural place
from which to cross-reference the documentation being asked for here.

_{`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 reading the manual sections on “specifying typical values,” “binary methods,” and “installing methods,” then compare the existing unary example with PR #3102’s assignment-method documentation. Document the binary form `X + X := X => (x,y) -> ...` and cross-reference it where appropriate so users can discover how typical values are declared for binary operators.

Written by the indexing model from the issue text.