Doc: Add examples and note convention of matching iteration

Author: mbauman

base/broadcast.jl

@@ -423,6 +423,23 @@ end
     broadcastable(x)
 
 Return either `x` or an object like `x` such that it supports `axes` and indexing.
+
+If `x` supports iteration, the returned value should match [`collect(x)`](@ref).
+
+# Examples
+```jldoctest
+julia> broadcastable([1,2,3]) # like `identity` since arrays already support axes and indexing
+3-element Array{Int64,1}:
+ 1
+ 2
+ 3
+
+julia> broadcastable(Int) # Types don't support axes, indexing, or iteration but are commonly used as scalars
+Base.RefValue{Type{Int64}}(Int64)
+
+julia> broadcastable("hello") # Strings break convention of matching iteration and act like a scalar instead
+Base.RefValue{String}("hello")
+```
 """
 broadcastable(x::Union{Symbol,AbstractString,Function,UndefInitializer,Nothing,RoundingMode,Missing}) = Ref(x)
 broadcastable(x::Ptr) = Ref{Ptr}(x) # Cannot use Ref(::Ptr) until ambiguous deprecation goes through