Update fact-upgrade-sharded-cluster-prereq.rst

Unless I'm missing something here, the issue we identified with v:0 indexes isn't related to upgrade at all, and is no more or less likely after the upgrade.  We definitely want to encourage index upgrade, but not doing so won't make things worse.

Mention this because seeing customer concern about this warning in the docs.

Signed-off-by: Sam Kleinman <samk@10gen.com>
Fix include doc

This "include" is not working.
See http://docs.mongodb.org/manual/tutorial/add-user-to-database/

I'm pretty sure it's because it's too close to the line above (looking at the other "include"(s) in other docs)

Signed-off-by: Sam Kleinman <samk@10gen.com>
minor: tweaking list of links to fit with the new list idiom
DOCS-1714: 2.4 upgrade index issue
DOCS-1041: removing query document to clarify example