Rollup merge of #140076 - aDotInTheVoid:jsondocline, r=GuillaumeGomez

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 rust-lang/rust#132748 (comment):

```
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``

Author: ChrisDenton