DOCS-1377 document missing isMaster fields and related edits

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1377 minor sphinx correction

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1377 edits incorporating hannes' comments

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1377 incorporating comments

Signed-off-by: Sam Kleinman <samk@10gen.com>
minor: add phrasing around hosts and arbiters arrays per hannes

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1377