Reword documentation for Service concept (#14526)

* Reword glossary entry for Service

New wording should be more suitable for inclusion via a
glossary_definition shortcode.

* Improve What's Next section for Service concept

* Rename Services concept to singular

This matches other concept documentation, eg Deployment.

* Use documentation IP address range

1.2.3.4 is outside the blocks reserved for documentation.

* Reword Service concept description

Services direct traffic to Pods, not containers

* Write IPVS in uppercase

Author: Tim Bannister