update documentation

testApp/cntmod.py

@@ -0,0 +1,8 @@
+class MySupport:
+  def __init__(self, rec, link):
+    pass
+  def detach(self, rec):
+    pass
+  def process(self, rec, reason):
+    rec.VAL = rec.VAL + 1
+build = MySupport

testApp/cntrec.db

@@ -0,0 +1,5 @@
+  record(longin, "test:count") {
+    field(DTYP, "Python Device")
+    field(INP , "@cntmod")
+    field(SCAN, "1 second")
+  }