document util.command_output return value change

After commit `30395911 util.command_output: return stderr, too`,
`command_output` returns a tuple of stdout and stderr. Document that change by
adding a changelog entry and add a usage note to `command_output`'s docstring.

beets/util/__init__.py

@@ -805,7 +805,8 @@ CommandOutput = namedtuple("CommandOutput", ("stdout", "stderr"))
 def command_output(cmd, shell=False):
     """Runs the command and returns its output after it has exited.
 
-    Returns a CommandOutput.
+    Returns a CommandOutput. The attributes ``stdout`` and ``stderr`` contain
+    byte strings of the respective output streams.
 
     ``cmd`` is a list of arguments starting with the command names. The
     arguments are bytes on Unix and strings on Windows.

docs/changelog.rst

@@ -74,6 +74,9 @@ For plugin developers:
   is almost identical apart from the name change. Again, we'll re-export at the
   old location (with a deprecation warning) for backwards compatibility, but
   might stop doing this in a future release.
+* ``beets.util.command_output`` now returns a named tuple of stdout and stderr
+  instead of only returning stdout. stdout can be accessed as a simple
+  attribute.
 
 For packagers: