chore: Improve unresolved xref messages

[filzrev]

This PR intended to improve output message that shown when unresolved xref id found.

Markdown content for test

<xref:AAA>
<xref href="BBB" data-throw-if-not-resolved="true"/> <!--This type of xref reference needs attribute to report unresolved xref -->
<a href="xref:CCC"/> 

Current output message

C:\temp\docfx_project\index.md: warning UidNotFound: 3 invalid cross reference(s) "<xref:AAA>", "BBB", "CCC".
C:\temp\docfx_project\index.md: Details for invalid cross reference(s): "<xref:AAA>" in line 7
C:\temp\docfx_project\index.md: Details for invalid cross reference(s): "" in line 0

First line show invalid cross references as expected.
But last line output "".

Improved output message

C:\temp\docfx_project\index.md: warning UidNotFound: 3 invalid cross reference(s) "<xref:AAA>", "BBB", "CCC".
C:\temp\docfx_project\index.md: Details for invalid cross reference(s): "<xref:AAA>" in line 7
C:\temp\docfx_project\index.md: Details for invalid cross reference(s): "BBB" in line 0, "CCC" in line 0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.98%. Comparing base (fe673ec) to head (ecd6a32).
Report is 132 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9884      +/-   ##
==========================================
+ Coverage   74.31%   78.98%   +4.66%     
==========================================
  Files         536      538       +2     
  Lines       23189    23316     +127     
  Branches     4056     4047       -9     
==========================================
+ Hits        17234    18415    +1181     
+ Misses       4853     3775    -1078     
- Partials     1102     1126      +24     

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[yufeih]

Thank you @filzrev !