`isvalid(::String)` is not defined but documented.

[yuyichao]

Ref #16590 (comment) and JuliaLang/IJulia.jl#441 (Opening an issue so that it is not lost).

I'm not sure what's the decision (I assume the method was deleted by accident?) but we should either fix the document (for isvalid(value) -> Bool) or add the method back.

[stevengj]

Note that we still have isvalid(String, s::String), so fixing this issue would be as simple as adding back isvalid(s::String) = isvalid(String, s) (or the more generic isvalid{T}(x::T) = isvalid(T, x) if we want to support this pattern for isvalid).