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

It isn't though - it behaves as any other backend does, namely, if you want to show a figure when *not in interactive mode* you have to do a ``plt.show()``. I was also careful with the wording, in that it does currently state ``...then subsequently shown``.