Working on simplifying the basic agent usage docs

Making it so the different agent usage docs inherit the same language.

Updated it so that the restart command shows that this is how you reload a config.
Merge branch 'restartagentdocupdate' of github.com:DataDog/documentation into restartagentdocupdate

Conflicts:
	content/guides/basic_agent_usage/osx.md
	content/guides/basic_agent_usage/redhat.md