Document AssertJ interoperability with Spock 2.4 and Groovy

Describe the bug
The feature from assertj/assertj#2520 seems to be incompatible with using the StringAssertfrom assertj-core in a Groovy and Spock setup.

The then block in a spock test feature wraps any assertion code line with an groovy powerassert assert under the hood expecting the actual assertion expression to be evaluated as a boolean value. In Groovy there is the org.codehaus.groovy.runtime.DefaultGroovyMethods#asBoolean(java.lang.Object) method that Spock is calling. Due to the changes made to the AbstractStringAssert class this method is inaccessible and returns a BooleanAssert instead.

• assertj core version: 3.25.3
• java version: 17
• groovy version: 4.0.18
• test framework version: Spock 2.3-groovy-4.0
• os (if relevant):

Test case reproducing the bug

Add a test case showing the bug that we can run

import spock.lang.Specification

import static org.assertj.core.api.Assertions.assertThat

class DemoSpec extends Specification {
    def 'assert some integer'() {
        when:
        def someInteger = 10

        then:
        assertThat(someInteger).isEqualTo(10)
    }

    def 'assert some string'() {
        when:
        def someString = 'some string'

        then:
        assertThat(someString).isEqualTo('some string')
    }
}