Precedence and associativity of the @ operator is undocumented

The `@` symbol is permitted as the first character of a custom operator.

```fs
let (@+) a b = a + b
1 @+ 2
```

https://github.com/fsharp/fslang-spec/blob/a3b1d61ae54eee380803cd6800c4d02c246bde92/spec/lexical-analysis.md?plain=1#L331

The precedence table, doesn't list the `@` operator to define its precedence and associativity.

https://github.com/fsharp/fslang-spec/blob/a3b1d61ae54eee380803cd6800c4d02c246bde92/spec/basic-grammar-elements.md?plain=1#L246-L284

The categorization table doesn't list `@` operator.

https://github.com/fsharp/fslang-spec/blob/a3b1d61ae54eee380803cd6800c4d02c246bde92/spec/basic-grammar-elements.md?plain=1#L190-L212

What would be the best way to determine this to update the spec?