[julia-users] Add documentation for overrides of Base functions?

[Sheehan Olver]

I tried adding the following

```julia
doc"""
    length(foo) -> Integer

returns the length of foo
"""
Base.length(foo::Foo)
```

inside a module, but it doesn't show up in 

```julia
?length
```

Any suggestions?  And is the `doc""" ..."""...` syntax documented somewhere?