12311 auto memory docs

Documentation PR for openmicroscopy/openmicroscopy#2595

This PR should initially be used for a clarification of the code in 2595, but should remain open for the likely numerous adjustments which will be necessary in the code. Built documentation should be visible under:

 * https://www.openmicroscopy.org/site/support/omero5-staging/sysadmins/server-advanced-configuration.html

Both the sections:
 * https://www.openmicroscopy.org/site/support/omero5-staging/sysadmins/unix/server-installation.html#jvm-memory-settings and
 * https://www.openmicroscopy.org/site/support/omero5-staging/sysadmins/windows/server-installation.html#jvm-memory-settings

have been updated to point to adv. configuration.
Worth adding an `omero.mem.percent` to the glossary above?