@_nodoc attribute for hiding symbols from the symbol graph

[taylorswift]

@QuietMisdreavus

It introduces a @_nodoc attribute to hide a symbol from the symbol graph (and thus from documentation) without marking it as internal or prefixing its name with an underscore. It also works on @_exported import statements, to hide symbols from the exported module.

instead of excluding the symbol (which can interfere with symbolgraph algorithms and have unintended side-effects by creating holes in the symbolgraph), could it instead emit a field like "visibility": "hidden" to indicate that the symbol should not be shown by the frontend?