[JUnit Platform] Update parallel execution documentation (#2609)

Clarify that Cucumber does not guarantee a maximum number of threads.

Closes: 2512

Author: mpkorstanje

cucumber-junit-platform-engine/README.md

@@ -185,14 +185,32 @@ in parallel is available as an opt-in feature. To enable parallel execution, set
 the `cucumber.execution.parallel.enabled` configuration parameter to `true`,
 e.g. in `junit-platform.properties`.
 
-Cucumber supports JUnit's `ParallelExecutionConfigurationStrategy`; see the 
-configuration options below.
+To control properties such as the desired parallelism and maximum parallelism,
+Cucumber supports JUnit 5s `ParallelExecutionConfigurationStrategy`. Cucumber
+provides two implementations: `dynamic` and `fixed` that can be set through
+`cucumber.execution.parallel.config.strategy`. You may also implement a `custom`
+strategy.
+
+* `dynamic`: Computes the desired parallelism as `<available cores>` * 
+`cucumber.execution.parallel.config.dynamic.factor`.
+
+* `fixed`: Set the desired parallelism to `cucumber.execution.parallel.config.fixed.parallelism`.
+
+* `custom`: Specify a custom `ParallelExecutionConfigurationStrategy`
+implementation through `cucumber.execution.parallel.config.custom.class`.  
+
+If no strategy is specified Cucumber will use the `dynamic` strategy with a
+factor of `1`.
+
+Note: Cucumber does not guarantee that the number of concurrently executing
+tests will not exceed the configured parallelism. The underlying `ForkJoinPool`
+used to execute scenarios may spawn additional threads. To obtain such
+guarantees us a custom parallel configuration strategy.
 
 ### Exclusive Resources ###
 
-The JUnit Platform supports parallel execution. To avoid flaky tests when
-multiple scenarios manipulate the same resource, tests can be
-[synchronized][junit5-user-guide-synchronization] on that resource.
+To avoid flaky tests when multiple scenarios manipulate the same resource, tests
+can be [synchronized][junit5-user-guide-synchronization] on that resource.
 
 [junit5-user-guide-synchronization]: https://junit.org/junit5/docs/current/user-guide/#writing-tests-parallel-execution-synchronization