Although `yes` and `no` are recognized as booleans by the pyyaml parser,
the correct keywords are `true` and `false` (as highlighted by the newly
added `truthy` rule).
This commit replaces the use of `yes`/`no` by `true`/`false` and
advertise it in the docs, but also makes sure this change is
backward-compatible (so that `yes` and `no` still work).