Refactor `@content` arguments specs

[jathak]

[connorskees]

Hello,

I've come across an interesting case when compiling Bootstrap that may be relevant here.

The minimized version is

@mixin foo($arg) {
  @include bar {
    color: $arg;
  }
}

@mixin bar {
  @include baz {
    @content;
  }
}

@mixin baz {
  @content;
}

@mixin font-size($value) {
  @include foo($value);
}

a {
  @include font-size(1rem);
}

which I suppose can be thought of as a triple chain of @content.