Fix misleading comments

See #450

exercises/107_threading.zig

@@ -75,10 +75,10 @@ pub fn main() !void {
     std.debug.print("Starting work...\n", .{});
 
     // These curly braces are very important, they are necessary
-    // to enclose the area where the threads are called.
+    // to enclose the area where the threads are called and joined.
-    // Without these braces, the program would not wait for the
+    // With these braces, the program will block and wait for all threads
-    // end of the threads and they would continue to run beyond the
+    // to finish right at the closing brace of this block, ensuring
-    // end of the program.
+    // "Zig is cool!" is always printed last.
     {
         // Now we start the first thread, with the number as parameter
         const handle = try std.Thread.spawn(.{}, thread_function, .{1});
@@ -102,8 +102,7 @@ pub fn main() !void {
         try io.sleep(std.Io.Duration.fromMilliseconds(400), .awake);
         std.debug.print("Some weird stuff, after starting the threads.\n", .{});
     }
-    // After we have left the closed area, we wait until
+    // The threads are guaranteed to be finished by the time we reach here.
-    // the threads have run through, if this has not yet been the case.
     std.debug.print("Zig is cool!\n", .{});
 }