reference: add doc for `Package`

We document the other py collectors, we should document `Package` as
well.

doc/en/reference/reference.rst

@@ -926,6 +926,12 @@ Parser
 .. autoclass:: pytest.Parser()
     :members:
 
+Package
+~~~~~~~
+
+.. autoclass:: pytest.Package()
+    :members:
+
 OptionGroup
 ~~~~~~~~~~~
 

src/_pytest/python.py

@@ -659,6 +659,9 @@ class Module(nodes.File, PyCollector):
 
 
 class Package(Module):
+    """Collector for files and directories in a Python packages -- directories
+    with an `__init__.py` file."""
+
     def __init__(
         self,
         fspath: Optional[LEGACY_PATH],