R has probably the best help feature for any language -- not only can you ask it about help on individual functions with "?", the tradition (and this continues with most add in packages as well as builtins) is not only does it give you info on the function, it gives you example code using it so you can understand what it does in practice.

Ruby has a similar help() function you can invoke from the REPL (irb), or from the CLI with the standalone tool `ri`[1]. Pretty nifty feature!

[1]: https://stackoverflow.com/a/25671404

Clojure has doc and source functions. For example:

  user=> (doc clojure.core)
  -------------------------
  clojure.core
    Fundamental library of the Clojure language

  user=> (doc +)
  -------------------------
  clojure.core/+
  ([] [x] [x y] [x y & more])
    Returns the sum of nums. (+) returns 0. Does not auto-promote
    longs, will throw on overflow. See also: +'

  user=> (source +)
  (defn +
    "Returns the sum of nums. (+) returns 0. Does not auto-promote
    longs, will throw on overflow. See also: +'"
    {:inline (nary-inline 'add 'unchecked_add)
    :inline-arities >1?
    :added "1.2"}
    ([] 0)
    ([x] (cast Number x))
    ([x y] (. clojure.lang.Numbers (add x y)))
    ([x y & more]
      (reduce1 + (+ x y) more)))

Like many others here I once wrote a toy lisp in golang, and I added a help function. It would give usage-information for all the built-in functions.

      > (help +)
      Arguments N arg1..argN
      Adds all arguments present to the first number.

      > (help map)
      Arguments lst:list fun:function
      Return a list with the contents of evaluating the given function on every item of the supplied list.
      See-also: map-pairs

utop for ocaml is cool, gives type signatures for functions. no explicit help, but does offer completions with tab.

Similarly, rtop for reason.

https://opam.ocaml.org/blog/about-utop/ and https://opam.ocaml.org/packages/rtop/

Although it's probably not at the top of many people's minds (includind mine) as a favorite programming language, Mathematica does have fantastic help, accessible with `?` and expandable from there as needed.

Smalltalk, by far, is the best in this category.