Update instructions for parallel-letter-frequency (#2713) (#2752)

* Update instructions for parallel-letter-frequency (#2713)

The exercise docs suggested that the submitted code is automatically
tested for the usage of parallelism. This is not the case, so the
paragraph rewritten to instead suggest students to check for parallelism
themselves.

To help, a few links to resources about concurrency and parallelism in
Java are added.

* Split up docs for parallel-letter-frequency (#2713)

The fact that the tests are currently not checking for parallelism is no
longer mentioned in the instructions.

Resources about concurrency are moved to `hints.md`.

* Apply suggestion to parallel-letter-frequency hints

Co-authored-by: Sander Ploegsma <sanderploegsma@gmail.com>

---------

Co-authored-by: Sander Ploegsma <sanderploegsma@gmail.com>

Author: yogan

exercises/practice/parallel-letter-frequency/.docs/hints.md

@@ -0,0 +1,7 @@
+# Hints
+
+The following resources may help you solve this exercise:
+
+- [Concurrency (Oracle Java Tutorials)](https://docs.oracle.com/javase/tutorial/essential/concurrency)
+- [Java Concurrency (Baeldung)](https://www.baeldung.com/java-concurrency)
+- [Java 8 Parallel Streams (Baeldung)](https://www.baeldung.com/java-8-streams#parallel-streams)

exercises/practice/parallel-letter-frequency/.docs/instructions.append.md

@@ -1,7 +1,3 @@
 # Instructions append
 
-Single-threaded (non-concurrent) solutions can pass all tests [but the last.](https://www.youtube.com/watch?v=mJZZNHekEQw)  Your solution will be tested for concurrency by submitting it as a [Runnable](https://docs.oracle.com/javase/7/docs/api/java/lang/Runnable.html) to an [ExecutorService.](https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html) Your solution must leverage multiple [Threads](https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html) to pass the final test.
-
-Java documentation on [parallel streams](https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html) may provide some help.
-
 As a stretch goal, consider if your implementation will work for characters with [diacritics or accents](https://en.wikipedia.org/wiki/Diacritic). For example, such solutions should not consider e and ë the same character. An example text for this case is [Wilhelmus](https://en.wikipedia.org/wiki/Wilhelmus), the Dutch national anthem.