Improve ExitCode docstring

Author: nicoddemus

src/_pytest/main.py

@@ -22,8 +22,9 @@
 
 class ExitCode(enum.IntEnum):
     """
-    encodes the valid exit codes of pytest
-    currently users may still supply other exit codes as well
+    Encodes the valid exit codes by pytest.
+
+    Currently users and plugins may supply other exit codes as well.
     """
 
     #: tests passed