[backport] Clarifies experimental / parallel example on manual.rst (#12472)

* Clarifies experimental / parallel on manual.rst

Details:
Calling `useParallel()` in example fails with compiler error
  Error: 'parallel' section without 'spawn'

Adding `spawn` causes error:
  Error: internal error: (filename: "ccgexprs.nim", line: 1032, column: 17)
  No stack traceback available
  To create a stacktrace, rerun compilation with ./koch temp c <file>

Therefore a separate proc, `threadedEcho`, is added for the echo'ing
of the string, which allows the example to build, however, `sync()`
must be added so that the "echo in parallel" strings will actually
be shown on the terminal. Otherwise, the program will spawn of the
threads and exit before they can return to the main thread.

* Fixes and clarifies example for threading in manual.rst

Issue:
Calling useParallel() in example failed with compiler error
`Error: 'parallel' section without 'spawn'`

Adding spawn yielded compiler error:
```bash
Error: internal error: (filename: "ccgexprs.nim", line: 1032, column: 17)
No stack traceback available
To create a stacktrace, rerun compilation with ./koch temp c
```

Proposed Solution:
- Separate proc, threadedEcho, is added for the echo'ing
  of the string, which allows the example to build
- Added the thread number so that it can demonstrate that sometimes
  threads which were started sooner, come back after threads which
  were started later.

(cherry picked from commit d731646106)

doc/manual.rst

@@ -6347,12 +6347,18 @@ is uncertain (it may be removed any time).
 Example:
 
 .. code-block:: nim
+  import threadpool
   {.experimental: "parallel".}
 
+  proc threadedEcho(s: string, i: int) =
+    echo(s, " ", $i)
+
   proc useParallel() =
     parallel:
       for i in 0..4:
-        echo "echo in parallel"
+        spawn threadedEcho("echo in parallel", i)
+
+  useParallel()
 
 
 As a top level statement, the experimental pragma enables a feature for the