Apply docs from pulp-2.4 that were missing.

This commit adds the docs changes that were present in the pulp-2.4
branch that were not present in the 2.4-release branch. The patch was
generated with this command:

$ git diff 2.4-release..pulp-2.4 docs/
Merge branch '2.4-release' into 2.4-testing
Merge branch '2.4-dev' into 2.5-testing