update libXt documentation on x.org website

The project page for libXt points to the specification:

but release 1.2.0 makes substantial updates (fixes for the conversion from nroff, as well as completing the conversion to use ANSI C).

I would like to get the updated documentation available from the project page.

For comparison, I have an updated version on my website here.

Here's a diffstat showing the amount of change:

$ diff -r -u specs /usr/build/libXt/libXt-20190621-1.2.0/specs|diffstat
 CH01.xml            |  692 ++++++++++++++++++++++++++--------------------------
 CH02.xml            |  282 ++++++++++-----------
 CH03.xml            |   40 +--
 CH04.xml            |  510 +++++++++++++++++++-------------------
 CH05.xml            |   25 +
 CH06.xml            |   43 +--
 CH07.xml            |  180 +++++++------
 CH08.xml            |   37 +-
 CH09.xml            |  431 ++++++++++++++++++--------------
 CH10.xml            |   76 +++--
 CH11.xml            |  226 ++++++++--------
 CH12.xml            |  291 +++++++++++----------
 CH13.xml            |  452 ++++++++++++++++++++++++++++++++-
 acknowledgement.xml |   73 ++++-
 appA.xml            |   33 +-
 appB.xml            |  175 ++++++-------
 appC.xml            |   99 +++----
 appD.xml            |    5 
 appE.xml            |   17 -
 appF.xml            |    3 
 intrinsics.xml      |   20 -
 libXt.ent.in        |only
 preface.xml         |    5 
 23 files changed, 2164 insertions(+), 1551 deletions(-)

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