Enable XML comments.

[fiseni]

Now that we have significant coverage of the code with XML comments, we may enable them (without this, the comments won't be packed in the Nuget package, and won't be available to the consumers). We have to add the following line in the project files

<DocumentationFile>bin\$(Configuration)\Ardalis.Specification.xml</DocumentationFile>

Once we add this, we'll get warnings if we missed adding an XML comment for any publicly visible construct.