XmlDoc fixes

[DedSec256]

Several bugs were found in XmlDocs:

1. In signature files, for 'new' type members xmldoc is not in member range

/// :( -- not in range
new: unit -> T

2. In signature files, for types without members everything is included in the range up to the next declaration

type A

///B -- in 'A' declaration range :(
type B = ...

3. For properties with explicit accessors XmlDoc range was incorrectly calculated in XmlDoc.Merge,
   when one of the xml docs is empty

This PR fixes them

[KevinRansom]

Thank you