clarify nest/hold ambiguity
DOCS-1957 fixing sleep command examples
DOCS-1918: fix to clarify index use in aggregation pipelines
Update mongostat.txt

Clarify that user must specify -p or --password without an argument to be prompted for a password interactively.

Signed-off-by: Sam Kleinman <samk@10gen.com>
additional clarification about the behavior of --password