Add note about atom order in selections

Add note about atom order in selections

Clarify the behavior of selections in that they always select atoms in
increasing order.  In the same context, mention the mechanisms in place
to work around this.  Use an example that shows the equivalence of
'a to b' and 'b to a' in the selection syntax.

Also clean up the formatting of the selection limitations help topic,
now that bullet lists are properly supported.

Related to #1742

Change-Id: I3daa17521767b3eaadcbc1d8c0c6fc986fa0aee3