Fallback to getproperty in at-doc

[tkf]

Before this PR

help?> Some(123).value
ERROR: MethodError: no method matching Base.Docs.Binding(::Some{Int64}, ::Symbol)
Closest candidates are:
  Base.Docs.Binding(::Module, ::Symbol) at docs/bindings.jl:12
Stacktrace:
 [1] top-level scope at none:0

After this PR

help?> Some(123).value
  Int64 <: Signed

  64-bit signed integer type.

I guess there are other possible behaviors (e.g., show field docstring if found) but I think getproperty is a good default fallback.

ref JuliaPy/PyCall.jl#635

[fredrikekre]

This could alternatively retrieve the fielddoc I guess?

[tkf]

Yeah, I can imagine extending `binding` to something like

binding(x, v) =
    if v isa Symbol && hasfielddoc(x, v)    # pseudo code
        fielddoc(x, v)                      # pseudo code
    else
        getproperty(x, v)
    end

but I don't know if there are APIs like hasfielddoc and fielddoc. REPL.fielddoc seems do it a bit differently. I thought falling-back to getproperty is a good start for extending it.

[tkf]

Actually, retrieving the doc in binding doesn't make sense as it should happen in Docs.doc, not Docs.binding. Maybe we should extend Binding from

struct Binding
    mod::Module
    var::Symbol
end

to

struct Binding
    thing  # allow any object, not just a `Module`
    var::Union{Symbol, String}
end

or maybe even to

struct Binding
    thing
    accessor # e.g., `:a`, `"b"`, `(:a, :b, :c)`
end

?