Modules: Rename FindXerces to FindXercesC

The FindXerces module was added in commit v3.1.0-rc1~155^2 (FindXerces:
New module to find Apache Xerces-C++, 2014-08-17).  However, there are
two implementations of Xerces, one in C++:

  http://xerces.apache.org/xerces-c/

and one in Java:

  http://xerces.apache.org/xerces-j/

Rename FindXerces to FindXercesC to clarify that it is about the C++
implementation.

While at it, add the missing CMake 3.1 release note about this module.

Suggested-by: Erik Sjölund <erik.sjolund@gmail.com>
Merge branch 'xerces-rename' into release