`default()` guard should be documented / mentioned with mixin documentation

[matthew-dean]

The default mixin guard is documented, but it's somewhat buried in the functions section.

I think, since the default function is actually not really a function, and is instead more "function-like", it would make more sense to move it to mixin documentation, with maybe a brief reference in the functions section.

To add

I think what could be added to this documentation is a little more explanation of when a default function applies, in relation to overloading. That is, it appears that default() is only applied once mixins have been filtered for arity.