Added 2.6.2 minor release notes

Also fixed link for 2.6.1 "All issues" (was copy&pasted from 2.4.10).

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-3606: edits
Added 2.6.2 changelog

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-3606: edits