docs for `copyto!` causes ambiguity errors

[longemen3000]

the broadcasting docs mention this optional method to define implace broadcast assignment:

Customizing broadcasting

Methods to implement	Brief description
...	...
Bypassing default machinery
Base.copyto!(dest, bc::Broadcasted{DestStyle})	Custom implementation of broadcast!, specializing on DestStyle
...	...

But defining copyto!(x<:Any,bc::Broadcasted{DestSyle}) causes ambiguity errors for any subsequent package that needs to do the same. i found this recently in StaticArrays + DataFrames JuliaArrays/StaticArrays.jl#1097 , where StaticArrays defines Base.copyto!(dest, B::Broadcasted{<:StaticArrayStyle}), causing ambiguity errors down the line, while following the docs.