Macro docs put the implementation after the description

[gamazeps]

This puts the description of marcos before their implementation.
It doesn't put the implementation at the bottom, but doing so while keeping the description on top would require a bigger rewrite of the file (as the description is currectly linked to the macro code, and separationg them would require creating another Item).
I'll look into it later
Closes #17616

[alexcrichton]

This isn't necessarily a local problem with just macros, no matter what order we place item definitions and documentation, there are pathological cases where both systems are difficult to understand. Right now "definition first then documentation" is consistent with the rest of the style that rustdoc uses. Additionally, bitflags! is a bit of an outlier in that most macros are not as large.

That being said, the navigability of rustdoc I think could always use some improvements, but I'm worried that this is locally optimizing for one outlier.

[gamazeps]

Oki dak :)

[aturon]

I'm going to go ahead and close this PR. Thanks for putting it together, though!