Update orderby.txt

cursor.sort() contains a warning about the 32MB limit on non-indexed sorting, this adds the same warning to $orderby - which is the underlying operator that powers sort().
The warning is largely a copy+paste with superficial name replacements.

Signed-off-by: kay <kay.kim@10gen.com>
tweak orderby size limit blurb
Reordered steps, added detail to step 2

Signed-off-by: kay <kay.kim@10gen.com>
reorg restore sharded clusters steps
trivial: swap step 5 and 6 in restore sharded cluster tutorial