chore: Add remarks to xmldoc for default constructor of HeaderApiVersionReader

It's easy to make a mistake expecting some defaults which are simply absent. I'd prefer getting rid of the default constructor, but that would be a breaking change.
It's worth to be more explicit in the docs, and explain, that headers must be set.

Author: voroninp

src/Common/src/Common/HeaderApiVersionReader.cs

@@ -15,6 +15,10 @@ public partial class HeaderApiVersionReader : IApiVersionReader
     /// <summary>
     /// Initializes a new instance of the <see cref="HeaderApiVersionReader"/> class.
     /// </summary>
+    /// <remarks>
+    /// There is no default header. Either initialize headers using the <see cref="HeaderNames"/> collection property,
+    /// or call a constructor that  accepts header names. Otherwise, this reader has no effect.
+    /// </remarks>
     public HeaderApiVersionReader() { }
 
     /// <summary>