frappy_psi.extparams.StructParam: fix doc + simplify

- the doc string contained wrong, obsolete information
+ simplify: remove customizeable mapping member name -> param name

Change-Id: I0ffdcb336f45001d45f68238b56e1e257d98dd3d
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/33993
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_extparams.py

@@ -116,12 +116,12 @@ def test_readonly():
             p = Parameter('control parameter p', FloatRange()),
             i = Parameter('control parameter i', FloatRange()),
             d = Parameter('control parameter d', FloatRange()),
-        ), {'p': 'pp', 'i':'ii', 'd': 'dd'}, readonly=True)
+        ), readonly=True)
 
     assert Mod.ctrlpars.readonly is True
-    assert Mod.pp.readonly is True
-    assert Mod.ii.readonly is True
-    assert Mod.dd.readonly is True
+    assert Mod.p.readonly is True
+    assert Mod.i.readonly is True
+    assert Mod.d.readonly is True
 
 
 def test_order_dependence1():