Tutorial: add info about non-blocking !() (#5449)

Closes #3394

### Motivation

After reading [the history of threading and
operators](#5444) I realized that
this was intended, to make the `!()` operator non-blocking. So let's
write about this in tutorial.

### Next steps

In the future we need to solve these points:
* Confusion around `.output` and `.out`.
* Show good cases where non-blocking is doing perfect work to people. Or
remove the non-blocking functionality [to remove
threading](#4710).

## For community
⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍
comment**

---------

Co-authored-by: a <1@1.1>
Co-authored-by: Gil Forsyth <gforsyth@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/tutorial.rst

@@ -387,23 +387,44 @@ input and output were redirected.  For example:
 
     >>> !(ls nonexistent_directory)
     CommandPipeline(
-        stdin=<_io.BytesIO object at 0x7f1948182bf8>,
-        stdout=<_io.BytesIO object at 0x7f1948182af0>,
-        stderr=<_io.BytesIO object at 0x7f19483a6200>,
         pid=26968,
         returncode=2,
         args=['ls', 'nonexistent_directory'],
         alias=['ls', '--color=auto', '-v'],
-        stdin_redirect=['<stdin>', 'r'],
-        stdout_redirect=[9, 'wb'],
-        stderr_redirect=[11, 'w'],
         timestamps=[1485235484.5016758, None],
         executed_cmd=['ls', '--color=auto', '-v', 'nonexistent_directory'],
         input=None,
         output=,
         errors=None
     )
 
+The captured object ``!()`` operator allows for non-blocking execution.
+You can call a long-running command, intersperse other commands and
+read the captured output later:
+
+.. code-block:: xonshcon
+
+    >>> p = !(echo snail)
+    >>> p.output
+    ''
+    >>> p.end()
+    >>> p.output
+    'snail'
+
+You can force ``xonsh`` to block and wait for the command to complete by asking for the return code,
+printing the object or reading the ``out`` attribute:
+
+.. code-block:: xonshcon
+
+    >>> p = !(echo snail)
+    >>> p.out
+    'snail'
+    >>> p = !(echo party)
+    >>> p.rtn
+    0
+    >>> p.output
+    'party'
+
 This object will be "truthy" if its return code was 0, and it is equal (via
 ``==``) to its return code. It also hashes to its return code. Converting the object
 to the string will return the output. This allows for some interesting new