Clarify fastcgi (rebased onto develop)


This is the same as gh-838 but rebased onto develop.

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Based on https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=7550
it seems there's some confusion on setting `omero.web.application_server` https://www.openmicroscopy.org/site/support/omero5/sysadmins/unix/install-web.html#quick-start. According to @chris-allan the default setting should work, so there's no need to set it.

Note I'm not sure what the situation is with Windows/IIS.