docs: Improve documentation for tasks (#2036)

Mention that they run in subprocesses.

Fixes #2035.

Author: InAnYan

docs/configuring.md

@@ -1563,7 +1563,8 @@ This allows you to keep separate the template metadata and the template code.
 
 Commands to execute after generating or updating a project from your template.
 
-They run ordered, and with the `$STAGE=task` variable in their environment.
+They run ordered, and with the `$STAGE=task` variable in their environment. Each task
+runs in its own subprocess.
 
 If a `dict` is given it can contain the following items:
 
@@ -1575,6 +1576,8 @@ If a `dict` is given it can contain the following items:
 If a `str` or `List[str]` is given as a task it will be treated as `command` with all
 other items not present.
 
+Refer to the example provided below for more information.
+
 !!! example
 
     ```yaml title="copier.yml"