Add a check for documented symbols that aren't public

[wilzbach]

From https://github.com/dlang/phobos/pull/5558

/// Add the mutable qualifier to the given type T.
package template MutableOf(T)     { alias MutableOf     =              T  ; }

Should yield a warning: "Documented symbol is not public".

[wilzbach]

A start -> https://github.com/dlang-community/D-Scanner/pull/585