Add `eldoc` support for Racket macros

[vkz]

Would be nice to have eldoc show something helpful when it comes to Racket macros. Probably impossible in general maybe there're some reasonable cases that we could cover.

Example:

(assert 'bla)
;; should show (assert arg) hint of some kind in the minibuffer

[greghendershott]

My quick reaction is that the difficulty in getting anything useful would vary dramatically based on how the macro is defined:

• syntax-rules: "trivial" / "helpful"
• syntax-case: "impossible" / "often useless"
• syntax-parse: "hard" / "possibly helpful"

In general I find eldoc of limited value for Racket. Partly that's because so much of Racket is syntax, i.e. this issue you filed. :) But not entirely. Having said that, I'd be delighted to take a pull request that improves this.