Add documentation of @ pattern in match
* `_` symbol -- the last special case is the `_` symbol, which is a wildcard
that will match any value without creating a binding.
+ While a symbol pattern will ordinarily match any value, the pattern `(@ <sym>)`,
+ where <sym> is any symbol, will attempt to match `x` against a value
+ already bound to `<sym>`, rather than matching and rebinding it.
+
Any other value pattern will only match if it is equal to `x`.
```
[x & cases]
