Update Elastic Agent command reference with Fleet Server flags (elast…

…ic#667)

* Update command reference with new fleet commands

* remove todo

* Add more detail about cert-related flags

* Mention install command in the line about the root user

Co-authored-by: Eric Davis <[email protected]>

* Make it clear that you are required to enroll after installing from deb/rpm

Co-authored-by: Eric Davis <[email protected]>

* Mention that installing as a service is more common than enrolling

* Clarify that the enroll command can be used to enroll an agent in a different policy

* Clarify what you can do with enroll command

* Change order of flags and optionality in the command sytnax

Co-authored-by: Eric Davis <[email protected]>

* Change order of flags and optionality in the command syntax

Co-authored-by: Eric Davis <[email protected]>

* Remove outdated info about passing username/password in URL

* Finalize command reference with updates from review

* Add links to related docs

* Apply suggestions from code review

Co-authored-by: Eric Davis <[email protected]>

* Apply additional suggestions from code review

Co-authored-by: Eric Davis <[email protected]>

* Remove redundant wording

Co-authored-by: Eric Davis <[email protected]>