improve Union docs as discused in discourse (#45109)

Co-authored-by: Viral B. Shah <ViralBShah@users.noreply.github.com>
Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>

base/docs/basedocs.jl

@@ -2857,23 +2857,33 @@ kw"Union{}", Base.Bottom
 """
     Union{Types...}
 
-A type union is an abstract type which includes all instances of any of its argument types. The empty
-union [`Union{}`](@ref) is the bottom type of Julia.
+A `Union` type is an abstract type which includes all instances of any of its argument types.
+This means that `T <: Union{T,S}` and `S <: Union{T,S}`.
+
+Like other abstract types, it cannot be instantiated, even if all of its arguments are non
+abstract.
 
 # Examples
 ```jldoctest
 julia> IntOrString = Union{Int,AbstractString}
 Union{Int64, AbstractString}
 
-julia> 1 isa IntOrString
+julia> 1 isa IntOrString # instance of Int is included in the union
 true
 
-julia> "Hello!" isa IntOrString
+julia> "Hello!" isa IntOrString # String is also included
 true
 
-julia> 1.0 isa IntOrString
+julia> 1.0 isa IntOrString # Float64 is not included because it is neither Int nor AbstractString
 false
 ```
+
+# Extended Help
+
+Unlike most other parametric types, unions are covariant in their parameters. For example,
+`Union{Real, String}` is a subtype of `Union{Number, AbstractString}`.
+
+The empty union [`Union{}`](@ref) is the bottom type of Julia.
 """
 Union