Fix help for .+ in the REPL

fixes #8903.

There is a slight oddity here (though not sure if this is related), this changes the current help e.g. for add:

```
julia> eval(parse("Base.@help(add)"))   # current help
add (generic function with 2 methods)

julia> eval(parse("Base.@help(\"add\")"))  # this PR
Base.Pkg.add(pkg, vers...)

   Add a requirement entry for "pkg" to "Pkg.dir("REQUIRE")" and
   call "Pkg.resolve()". If "vers" are given, they must be
   "VersionNumber" objects and they specify acceptable version
   intervals for "pkg".
```
(Note: Not sure how this small change fits in with docile/md upheaval #8588 #8791?)