update // docstring (#52050)

The `//` operator supports non-real numbers, so updating the docstring
would be helpful.

Author: hyrodium

base/rational.jl

@@ -47,6 +47,12 @@ end
     //(num, den)
 
 Divide two integers or rational numbers, giving a [`Rational`](@ref) result.
+More generally, `//` can be used for exact rational division of other numeric types
+with integer or rational components, such as complex numbers with integer components.
+
+Note that floating-point ([`AbstractFloat`](@ref)) arguments are not permitted by `//`
+(even if the values are rational).
+The arguments must be subtypes of [`Integer`](@ref), `Rational`, or composites thereof.
 
 # Examples
 ```jldoctest
@@ -55,6 +61,13 @@ julia> 3 // 5
 
 julia> (3 // 5) // (2 // 1)
 3//10
+
+julia> (1+2im) // (3+4im)
+11//25 + 2//25*im
+
+julia> 1.0 // 2
+ERROR: MethodError: no method matching //(::Float64, ::Int64)
+[...]
 ```
 """
 //(n::Integer,  d::Integer) = Rational(n,d)