Add documentation for custom channel sync checkboxes

[r-richardson]

Documentation Update Required

The docs do not include any information about the optional checkboxes found in the webui for syncing channel repos.

What does the documentation say now?

Currently the optional checkboxes are not mentioned anywhere within the docs.

What change is required?

There should be documentation on what the options do, and how they compare to the default sync behaviour.

[r-richardson]

It is also worth mentioning that these flags will be saved persistently for each channel. Note that the "Sync now" button also saves the channel flags before performing the sync run.

[keichwa]

@r-richardson Hi Robert, if wanted you can review https://github.com/uyuni-project/uyuni-docs/pull/2896 now.