[mdoc] Fix export-msxdoc regression, updated expected output.

Commit 44b2b4a8 completely broke `mdoc export-msxdoc` because
EcmaDoc.GetCref() was removed. Expected:

	<member name="C:Mono.DocTest.DocAttribute(System.String)">

Actual:

	<member name="Mono.DocTest.DocAttribute..ctor">

i.e. useless.

The fix: bring back GetCref(), this time within monodocs2slashdoc.cs.

Commit 0ad6ffde changed the HTML-generating stylesheet and moved some
@id attributes around, thus breaking the mdoc unit tests (expected
output likewise changed). Fix the monodoc XSLTs so that we don't emit
id="" attributes (id's with no value).

Update mdoc's expected output so that tests pass[*].

[*]: tests will NOT actually pass, as 04bf9add -- attempted fix for
an OS X stackoverflow caused by 3d3fe6af -- results in a gigantic
216MB diff because String.get_Chars() is repeated over and over and
over... The updated expected output in this commit is the "ideal"
update needed after locally reverting 3d3fe6af and 04bf9add.)