Update documentation URLs to use 2.4-release.

We don't build pulp-2.4 anymore, so I've updated all documentation URLs
to use the new 2.4-release branch name.
Grammar fixes.
Merge branch '2.4-testing' into 2.4-dev
Merge branch '2.4-dev' into 2.5-testing
Merge branch '2.5-testing' into 2.5-dev
Merge branch '2.5-dev'