Clarify default channel behavior when no channels field is specified in ImageSetConfiguration

Summary

The documentation for oc-mirror v2 operator catalog filtering (module oc-mirror-operator-catalog-filtering.adoc) does not explicitly state that when no channels: field is specified for an operator package in the ImageSetConfiguration, all channels of that package are included in the mirroring set.

Current wording (Scenario 3)

The current text reads:

"One bundle, corresponding to the head version for each channel of that package."

While this is technically accurate, the phrase "each channel" only implies that all channels are included. Users have to read carefully to understand this — it is easy to misinterpret as meaning only the default channel is mirrored.

Suggested improvement

Add an explicit clarification, for example:

"When no channels field is specified for a package, oc-mirror includes all channels of that package and mirrors the head bundle from each one. To mirror only specific channels, add a channels block listing the desired channel names."

This would help users understand the potentially large scope of an unfiltered package specification and make an informed decision about whether to add explicit channel filters.

Context

This came up during a support discussion where the behavior was initially misunderstood. The oc-mirror team confirmed that the default is to mirror all channels when no channels: field is specified.

Open oc-mirror-operator-catalog-filtering.adoc and locate Scenario 3. Read the surrounding operator catalog filtering guidance and update the wording so the no-channels case explicitly states that all channels are included and explains how to select specific channels. Done means the default scope and filtering option are unambiguous.