exec-task description

[eval]

Is your feature request related to a problem? Please describe.

Given the following tasks:

;; bb.edn
:tasks {
  foo clojure.core/prn
  bar (exec 'clojure.core/prn)
}

...bb tasks will show a description for foo but not for bar:

$ bb tasks
The following tasks are available:

foo Prints the object(s) to the output stream that is the current value
bar

Describe the solution you'd like

The description of an 'exec-task' is extracted from the docstring just like for a qualified symbol.

Describe alternatives you've considered
It seems wrapping the exec-form in e.g. {:doc "duplicated doc" :task (exec ,,,,)} is currently the way to go.