Updated whats new for nbagg and legend/patheffects docs.

I've put this against 1.4.x rather than 1.4.0-doc since the v1.4.1 release is so imminent, but I'll happily change if needs be.
This should probably explicitly say that an explicit call to `plt.show()` is required (because we don't yet have the hooks set up right with ipython).  See discussion at https://github.com/ipython/docker-notebook/pull/7#issuecomment-55449631