jsondocck: Require command is at start of line
In one place we use `///``@``` instead of `//``@`.`` The test-runner allowed it, but it probably shouldn't. Ran into by ``@lolbinarycat`` in https://github.com/rust-lang/rust/pull/132748#issuecomment-2816469322:
```
error: unknown disambiguator `?(`
##[error] --> /checkout/tests/rustdoc-json/fns/return_type_alias.rs:3:25
|
3 | ///@ set foo = "$.index[?(``@.name=='Foo')].id"``
| ^^
|
```
Maybe it's also worth erroring on this like we added in #137103
r? ``@GuillaumeGomez``
df9e15e69f