add note for omero.jvmcfg.* setters about admin jvmcfg (rebased from develop)

... readers of that page might not notice that `bin/omero admin jvmcfg` is useful to check the results of their settings changes. Staged at https://www.openmicroscopy.org/site/support/omero5-staging/sysadmins/server-performance.html#tips.

--rebased-from #995