Split up docs for parallel-letter-frequency (exercism#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`.

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

@@ -0,0 +1,7 @@
+# Hints
+
+The following resources may help you with this:
+
+- [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

@@ -1,11 +1,3 @@
 # Instructions append
 
-The automated tests are currently not checking for parallelism. Please ensure yourself that your solution is using some form of parallelism, like threads or parallel streams.
-
-The following resources may help you with this:
-
-- [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)
-
 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.