How can user defined functions (say `f`) have meaningful printouts when inspected via the REPL using `?f`or `help(f)`

For example imagine I write the following funciton

```
function f(x::Float64, y::Float64)
    return 2x - y^2
end

```

If I load this into a julia session and try `help(f)` I get the following:

```
julia> help(f)
f (generic function with 1 method)

```

What if instead I wanted to see something like

```
julia> help(f)
f

   Compute 2 times x minus y squared

```

where the description "Compute 2 times x minus y squared" is written somewhere. I am guessing the answer to my question can be determined from the answer to the question "Where is the somewhere the description should be written?"

---

By way of example, if I wanted to do the same in python, I could define the function and put the description as a docstring:

```
def f(x, y):
    """
    Compute 2 times x minus y squared
    """
    return 2 *  x - y ** 2

```

which would make my description immediately available when I type `help(f)` or `f?` from IPython.