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.
I managed to confuse myself over `max_total`, `min_total` and `use_total`. Could you change `max_total` to something along the lines of `Maximum amount of memory that OMERO will use, subject to use_total and omero.mem.strategy`?