Improve wording of auth strategy mode/default

I found this wording confusing, and noticed other people have as well (e.g. dwyl/hapi-auth-jwt2#137 ).  I think the proposed update better reflects when `server.auth.strategy` sets a default strategy for routes.

API.md

@@ -649,7 +649,7 @@ Registers an authentication strategy where:
 - `name` - the strategy name.
 - `scheme` - the scheme name (must be previously registered using
   [`server.auth.scheme()`](#serverauthschemename-scheme)).
-- `mode` - if `true`, the scheme is automatically assigned as a required strategy to any route
+- `mode` - unless set to `false`, the scheme is automatically assigned as the default strategy for any route
   without an `auth` config. Can only be assigned to a single server strategy. Value must be `true`
   (which is the same as `'required'`) or a valid authentication mode (`'required'`, `'optional'`,
   `'try'`). Defaults to `false`.