Update SA1642 (ConstructorSummaryDocumentationMustBeginWithStandardText) to handle included documentation

[sharwell]

The issue described in #1603 applies to SA1642 as well.

[peteroupc]

This issue appears to be reappearing for me, in that this check seems not to expand included documentation even if that documentation contains the required text.

Here is a class which exhibits this issue.

namespace MyNamespace {
  public class MyClass {
    /// <include file='docs.xml'
    /// path='docs/doc[@name="MyClassCons"]/*'/>
    public MyClass(int x){ }
  }
}

Note that the include file references a file called docs.xml, which is as follows for the purposes of this report:

<docs><doc name="MyClassCons">
<summary>Initializes a new instance of the <see cref='MyClass' /> class.</summary></doc></docs>

[sharwell]

@peteroupc Can you create a new issue for this?