This PR removes the assertion around when the 'task' field of
a check may be set. Starting in Nomad 1.4 we automatically set
the task field on all checks in support of the NSD checks feature.
This is causing validation problems elsewhere, e.g. when a group
service using the Consul provider sets 'task' it will fail
validation that worked previously.
The assertion of leaving 'task' unset was only about making sure
job submitters weren't expecting some behavior, but in practice
is causing bugs now that we need the task field for more than it
was originally added for.
We can simply update the docs, noting when the task field set by
job submitters actually has value.
This PR updates the checks documentation to mention support for checks
when using the Nomad service provider. There are limitations of NSD
compared to Consul, and those configuration options are now noted as
being Consul-only.
This PR creates a top-level 'check' page for job-specification docs.
The content for checks is about half the content of the service page, and
is about to increase in size when we add docs about Nomad service checks.
Seemed like a good idea to just split the checks section out into its own
thing (e.g. check_restart is already a topic).
Doing the move first lets us backport this change without adding Nomad service
check stuff yet.
Mostly just a lift-and-shift but with some tweaked examples to de-emphasize
the use of script checks.
