Transactional Memory documentation in extend.texi

[Torvald Riegel]

We are aware that the TM language constructs should be documented in
extend.texi.  However, the most recent public version of the C++ TM
specification document is outdated, and a new version is supposed to be
released in a few weeks.

Therefore, we'd like to wait until the release of the new specification
document so that we can just cite this new document instead of having to
cite the old one and list all the changes.

Meanwhile, I have updated the GCC wiki page on TM to be a bit more
verbose.

Torvald