Add missing Zip API docs

[maryamariyan]

cc: @stephentoub @carlossanlop

Here are the two APIs with missing docs:

namespace System.Linq
{
    public static class Enumerable
    {
        public static IEnumerable<ValueTuple<TFirst, TSecond>> Zip<TFirst, TSecond>(this IEnumerable<TFirst> first, IEnumerable<TSecond> second);
    } 

    public static class Queryable
    {
        public static IQueryable<ValueTuple<TFirst, TSecond>> Zip<TFirst, TSecond>(this IQueryable<TFirst> source1, IEnumerable<TSecond> source2);
    }
}

And sample tests in corefx showing their usages:
https://github.com/dotnet/corefx/blob/master/src/System.Linq/tests/ZipTests.netcoreapp.cs
https://github.com/dotnet/corefx/blob/master/src/System.Linq.Queryable/tests/ZipTests.netcoreapp.cs

[mairaw]

Looks good overall @maryamariyan. Left some comments/questions in addition to @stephentoub's comments.

[mairaw]

Language-wise LGTM. @stephentoub good to merge?