Minor improvements to webapi.rst.

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Add comments and a caveat in webapi.rst indicating that
the needs-rebalancing field may be computed incorrectly. refs #1115, #1784, #1477

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>