1.7.0: shipping rule "Only" reserves a method, "Show" no longer does

Since 1.6.0 "Show" both whitelisted a method for its group and hid it from
everybody else. A group shown ordinary methods next to a free one therefore
took the ordinary ones away from every stranger, who was left with only the
free method. "Only" now carries the reservation; "Show" is just the group's
whitelist. Existing show rules are converted to only once on update so the
checkout does not change until the shop chooses.

Co-Authored-By: Claude Code <noreply@anthropic.com>

docs/README.md

@@ -1,6 +1,6 @@
 # Nolimit Discounts
 
-Version 1.6.3 · a WooCommerce plugin · slug `woocommerce-email-discount-manager`
+Version 1.7.0 · a WooCommerce plugin · slug `woocommerce-email-discount-manager`
 
 Gives a customer a discount, and decides which shipping methods they may
 choose, based on the email address they check out with.
@@ -29,11 +29,14 @@ address, then domain, then fragment.
 A discount group can be given shipping rules, which is the half worth reading
 twice because it decides what a stranger sees.
 
-A method a group is told to **show** is *reserved* for that group. Customers
+A method a group is set to **only** is *reserved* for that group. Customers
 who match no group do not see it. Before 1.6.0 a method set up for approved
 company addresses stayed on the checkout for everybody, so free delivery meant
 for one customer could be chosen by anyone who found it — the bug that release
-fixed.
+fixed. **Show** is the weaker rule and reserves nothing: it offers the method
+to the group and leaves everyone else alone. 1.6.0 made show do both, which
+took ordinary methods away from strangers whenever they were listed beside a
+free one; 1.7.0 split them and turned every old show rule into only.
 
 The plugin never leaves a customer with no shipping method at all. If applying
 the rules would empty the list, every method comes back.