bevy_reflect: Update internal docs regarding anonymous function type names

[MrGVSV]

Objective

As pointed out by @SkiFire13 on Discord, I was incorrect in #14641 regarding the type name of anonymous functions. I had stated that they will return something like fn(i32, i32) -> i32, but this is wrong. They actually behave like closures (despite not technically being closures) and return something more like foo::bar::{{closure}}.

This isn't a major issue because the reasoning behind #14641 still stands. However, the internal documentation should probably be updated so future contributors don't believe the lies I left behind.

Solution

Updated the internal documentation for create_info to reflect the actual type name of an anonymous function.

In that same module, I also added a test for function pointers and updated all tests to include sanity checks for the std::any::type_name of each category of callable.

Testing

You can test locally by running:

cargo test --package bevy_reflect