Update docs related to parallel execution.

README.md

@@ -215,16 +215,19 @@ $sites['leo.drall.local'] = 'leo';
 
 This puts the sites `donnie` and `leo` in a group named `bluish`.
 
-## Parallel execution (Experimental)
+## Parallel execution
 
 Say you have 100 sites in a Drupal installation. By default, Drall runs
 commands on these sites one after the other. To speed up the execution, you
 can ask Drall to execute multiple commands in parallel. You can specify the
 number of workers with the `--drall-workers=n` option, where `n` is the
 number of processes you want to run in parallel.
 
-Please keep in mind that running a high number of workers can create problems!
-The maximum number of workers allowed is set to `10` at the moment.
+Please keep in mind that the performance of the workers depends on your
+resources available on the computer executing the command. If you have low
+memory, and you run Drall with 4 workers, performance might suffer. Also,
+some operations need to be executed sequentially to avoid competition and
+conflict between the Drall workers.
 
 ### Example: Parallel execution