Be able to add to a member's docs from a distance

[Hixie]

Sometimes there are docs of this style:

/// Foo bar.
///
/// Used by the [baz] to do [quux].
///
/// Bla bla.
Bar foo();

It would be nice if instead the docs looked like this:

/// Foo bar.
///
/// @inserts
///
/// Bla bla.
Bar foo();

// ...

/// The baz of the quux.
///
/// @insert in foo: Used by the [baz] to do [quux].
void baz();

This would help with the problem where baz changes later to not use foo, and now we have out of date stale documentation.

This is based on an idea by @yjbanov .