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>
docs/nodekeys.rst: explain what node keys are for, with compatibility notes
NEWS: add entry for ref #166

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Add "[global-opts]" to help synopsis for tahoe ls. refs #166

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
known_issues: update chart-API text, with suggestions from Leif. refs #1942
known_issues.rst: get closer to real date
update some copyright dates