Clarify pytest_ignore_collect docs

Fixes #12383

src/_pytest/hookspec.py

@@ -311,7 +311,12 @@ def pytest_collection_finish(session: "Session") -> None:
 def pytest_ignore_collect(
     collection_path: Path, path: "LEGACY_PATH", config: "Config"
 ) -> Optional[bool]:
-    """Return True to prevent considering this path for collection.
+    """Return ``True`` to prevent considering this path for collection.
+
+    Return ``None`` to let other plugins ignore the path for collection.
+
+    Returning ``False`` will forcefully consider this path to be considered
+    for collection, without giving a chance of other plugins to ignore this file.
 
     This hook is consulted for all files and directories prior to calling
     more specific hooks.