Clarifies experimental / parallel example on manual.rst

[UNIcodeX]

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

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.

[UNIcodeX]

Observation: Sometimes without sync() all threads echo as expected, and sometimes they don't. I'm unsure if this is an issue with my machine(s).

It may be good, for illustrative purposes of the example, to actually expand a bit further and output the thread number, illustrating that threads spawned later may return before threads spawned earlier.

import threadpool, strformat
{.experimental: "parallel".}

proc threadedEcho(s: string, i: int) =
  echo(fmt"'{s}' for thread # {i}")

proc useParallel() =
  parallel:
    for i in 0..4:
      spawn threadedEcho("echo in parallel", i)
  sync()

useParallel()

Which, in my testing, yields something like this:

'echo in parallel' for thread # 0
'echo in parallel' for thread # 2
'echo in parallel' for thread # 1
'echo in parallel' for thread # 3
'echo in parallel' for thread # 4

[Araq]

parallel does not require sync! If it doesn't work, that's a bug, sync is wrong here.

[UNIcodeX]

I've ran it several more times without the sync() and did not have the issue. Chalking that one up to some weirdness.

What about the rest??

[UNIcodeX]

Removed sync().

Added output of thread number.

import threadpool
{.experimental: "parallel".}

proc threadedEcho(s: string, i: int) =
  echo(s, " ", $i)

proc useParallel() =
  parallel:
    for i in 0..4:
      spawn threadedEcho("echo in parallel", i)

useParallel()

example output:

echo in parallel 0
echo in parallel 3
echo in parallel 4
echo in parallel 1
echo in parallel 2