allow doc literals in let blocks to omit empty bindings, as comments

[aryairani]

Is your feature request related to a problem? Please describe. We don't have a syntax for comments within a definition (// comments are thrown away by the parser), but we sometimes use string constants instead.

foo = 
  Ability1.blarp
  _ = "Here comes the fancy bit."
  Ability2.bazbazbaz 6.54

Describe the solution you'd like

foo = 
  Ability1.blarp
  {{Here comes the fancy bit.}}
  Ability2.bazbazbaz 6.54

And then this Doc could also, optionally, be pretty-printed in the GUI or when a definition is displayed.

Describe alternatives you've considered Figure out some other first-class comments/internal documentation syntax, and/or choose something that will work to document a complex type or ability.

Additional context