Documentation update for oc-mirror catalog opm limitation (ppc64le)

There is a feature of oc-mirror, where operator images are mirrored from one registry to another, the code extracts the opm tool to rebuild the catalogs source: rebuildCatalogs and source: processCatalogRefs. 

This means if the opm that is used in the catalog is ppc64le and you are on intel and run oc-mirror the process will not function as expected and say there is a problem exec'ing opm.

So, there is a limitation... I'll try to summarizeIf the source catalog bundles an architecture specific opm, you should execute the mirroring from that architecture. For instance, if you mirror a ppc64le catalog, you should run from a power machine. If you run from another architecture

4.14 and higher

Everytime, and works as expected

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