[ FLOC-1010] Improvements to the release process documentation

Fixes https://clusterhq.atlassian.net/browse/FLOC-1010
Let's make the naming consistent with `gettinginvolved/infrastructure/release-schedule-versioning`. The release names are "Major Marketing Release", "Minor Marketing Release", "Weekly Development Release" and "Pre-release". Please make this sentence and anywhere else in this document consistent with those. I concede that the explanation for  "Weekly Development Release" includes "Weekly releases..." but it still takes some thinking.