Update documentation and NEWS for xtree concept.

Update documentation and NEWS for xtree concept.

Final patch of the xtree serie, which provides the documentation.

The xtree concept was committed in the revisions
16120 : Support pool of unique string in pub_tool_deduppoolalloc.h
16121 : Implement a cache 'address -> symbol name' in m_debuginfo.c
16122 : Add VG_(strIsMemberXA) in pub_tool_xarray.h
16123 : Addition of the pub_tool_xtree.h and pub_tool_xtmemory.h modules, and of the --xtree-memory* options
16124 : Addition of the options --xtree-memory and --xtree-memory-file
16125 : Small changes in callgrind_annotate and callgrind manual
16126 : Locally define vgPlain_scrcmp in 2 unit tests
16127 : Support for xtree memory profiling and xtmemory gdbsrv monitor command in helgrind
16128 : Support for xtree memory profiling and xtmemory gdbsrv monitor command in memcheck
16129 : Update massif implementation to xtree

Some smaller follow-up patches to be expected to add some regtests,
and refine documentation.

Thanks to Ivo, Julian and Josef for the review comments.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16131