Document select.epoll.register allowlist entries (#12159)

stdlib/@tests/stubtest_allowlists/linux-py38.txt

@@ -1,3 +1,5 @@
+# `eventmask` argument exists at runtime, but is not correctly recognized
+# while being inspected by stubtest. Fixed in Python 3.10.
 select.epoll.register
 
 # Exists at runtime, but missing from stubs

stdlib/@tests/stubtest_allowlists/linux-py39.txt

@@ -1,3 +1,5 @@
+# `eventmask` argument exists at runtime, but is not correctly recognized
+# while being inspected by stubtest. Fixed in Python 3.10.
 select.epoll.register
 
 # Exists at runtime, but missing from stubs