No documentation about repository filters for reposync (out of the CLM context)

[meaksh]

Uyuni is currently missing any documentation around adding filters spacewalk-repo-sync on custom repositories. To clarify, these filters are NOT related with CLM filters.

This existing feature on Uyuni can be used via:

• web UI (for custom repositories)
• XMLRPC API
• spacecmd
• spacewalk-repo-sync CLI

Even if the usage of these filters are not really common, maybe we should consider adding some documentation about this in the Uyuni documentation.