Add hint for :help to REPL.REPLCompletions.UndefVarError_hint (JuliaLang#41754)

Author: mkitti

stdlib/REPL/src/REPLCompletions.jl

@@ -907,6 +907,10 @@ function UndefVarError_hint(io::IO, ex::UndefVarError)
         print(io, "\nsuggestion: Use `||` for short-circuiting boolean OR.")
     elseif var === :and
         print(io, "\nsuggestion: Use `&&` for short-circuiting boolean AND.")
+    elseif var === :help
+        println(io)
+        # Show friendly help message when user types help or help() and help is undefined
+        show(io, MIME("text/plain"), Base.Docs.parsedoc(Base.Docs.keywords[:help]))
     end
 end