DOCS-1228 mongoexport:clarify that --csv requires fields to be specified

Signed-off-by: Sam Kleinman <samk@10gen.com> with amendments.
minor copy edits

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1272: removing redacted setting
DOCS-1277 hint with index name

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1185 clarify _id and projection doc

Signed-off-by: Sam Kleinman <samk@10gen.com>
diff-projection