Try to make private registry FAQ less confusing

[johnraz]

In some environments, authorisation provided by the platform is
used instead of image pull secrets. Google Container Registry works
this way, for example (and we have introduced a special case for it
so Flux will work there too)

This sentence drove me in the wrong direction, I thought GCR wouldn't work with imagePullSecrets at all while I believe the "special case for it" meant that it actually IS working with imagePullSecrets ... I hope my change makes things a bit clearer for others that may be confused in the same way.