Clarify switch between price set and manual membership (CRM-12575)

When site admins create a new membership, they don't often understand the distinction between the "old" way of choosing the membership type versus using a price set.  They'll try things out, and once they choose a price set, it's not at all clear that the way to go back to the other method and not use a price set is to select "Choose a price set".

This is a small change to the buildAmount function that's called when the price set field changes.  It rewrites the option "Choose price set" to "Manual membership and price" when a price set is selected, and it sets it back to "Choose price set" when none is selected.

In the process, I added a piece to the field description explaining that you can choose to pick a price set instead of membership organization and type.  A ton of first-timers don't get that, and they select an organization, type, and price set each time.