A few minor fixes to the upgrading guide

This draws on a small number of the issues outlined in #249. Thanks to
@MichaelBanks.
Reworded the generator changes section

This should make things clearer to read.
Tweaked the removed generators section
Merge pull request #250 from GrahamCampbell/2.0-docs

[2.0] Improved The Upgrade Guide
Merge remote-tracking branch 'origin/2.0'