Clarify wording of `podTemplate` documentation

[jon-signal]

What would you like to be added/changed?

As discussed in a separate thread, the phrasing of the documentation for podTemplate in ProcessSettings is a little confusing. It currently reads:

If a container image with a tag is specified the operator will throw an error and stop processing the cluster.

This is meant to refer narrowly to the main container and sidecar, which are controlled by the operator, but readers might conclude from this line that they may not specify any additional containers.