docs: cleanup the explanation paragraphs at the start of each exercise.

exercises/threads/threads1.rs

@@ -1,10 +1,12 @@
 // threads1.rs
-// Execute `rustlings hint threads1` or use the `hint` watch subcommand for a hint.
-
-// This program spawns multiple threads that each run for at least 250ms,
-// and each thread returns how much time they took to complete.
-// The program should wait until all the spawned threads have finished and
-// should collect their return values into a vector.
+//
+// This program spawns multiple threads that each run for at least 250ms, and
+// each thread returns how much time they took to complete. The program should
+// wait until all the spawned threads have finished and should collect their
+// return values into a vector.
+//
+// Execute `rustlings hint threads1` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE
 

exercises/threads/threads2.rs

@@ -1,7 +1,11 @@
 // threads2.rs
-// Execute `rustlings hint threads2` or use the `hint` watch subcommand for a hint.
-// Building on the last exercise, we want all of the threads to complete their work but this time
-// the spawned threads need to be in charge of updating a shared value: JobStatus.jobs_completed
+//
+// Building on the last exercise, we want all of the threads to complete their
+// work but this time the spawned threads need to be in charge of updating a
+// shared value: JobStatus.jobs_completed
+//
+// Execute `rustlings hint threads2` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE
 
@@ -27,8 +31,9 @@ fn main() {
     }
     for handle in handles {
         handle.join().unwrap();
-        // TODO: Print the value of the JobStatus.jobs_completed. Did you notice anything
-        // interesting in the output? Do you have to 'join' on all the handles?
+        // TODO: Print the value of the JobStatus.jobs_completed. Did you notice
+        // anything interesting in the output? Do you have to 'join' on all the
+        // handles?
         println!("jobs completed {}", ???);
     }
 }

exercises/threads/threads3.rs

@@ -1,5 +1,7 @@
 // threads3.rs
-// Execute `rustlings hint threads3` or use the `hint` watch subcommand for a hint.
+//
+// Execute `rustlings hint threads3` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE