Port System.Formats.Tar and System.Data undoc APIs from Preview7 (#10294)

* Port System.Formats.Tar.TarEntry.DataOffset
* Port System.Data.SqlDbType.Json

Author: carlossanlop

xml/System.Data/SqlDbType.xml

@@ -603,7 +603,7 @@
       </ReturnValue>
       <MemberValue>35</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>A JSON value.</summary>
       </Docs>
     </Member>
     <Member MemberName="Money">

xml/System.Formats.Tar/TarEntry.xml

@@ -69,9 +69,12 @@
         <ReturnType>System.Int64</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
-        <remarks>To be added.</remarks>
+        <summary>Gets the starting position of the data stream respective to the archive stream.</summary>
+        <value>A number that represents a position in a stream.</value>
+        <remarks>
+          <para>If the entry does not come from an archive stream or if the archive stream is not seekable, returns -1.</para>
+          <para>The position value returned by this property is relative to the absolute start of the archive stream, independent of where the tar archive begins.</para>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="DataStream">