Update OMERO.grid doc (rebased onto develop)



This is the same as gh-678 but rebased onto develop.

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This PR fixes deficiencies in the OMERO.grid deployment document and adds a section related to multi-host grid deployment.

To test:
- check spelling, syntax, general readability,
- (for the more daring) verify that multi-host deployment works based on the given example.

/cc @joshmoore, @dpwrussell