more precise wording, typos

Change-Id: I6ca052fa512b5864b1ce29a22808a432c6f527db

secop_psi/historywriter.py

@@ -70,11 +70,11 @@ class FrappyHistoryWriter(frappyhistory.FrappyWriter):
           - period:
               the typical 'lifetime' of a value.
               The intention is, that points in a chart may be connected by a straight line
-              when the distance is lower than this value. If not, the line should be drawn
-              horizontally from the last point to a point <period> before the next value.
+              when the distance is lower than twice this value. If not, the line should be
+              drawn horizontally from the last point to a point <period> before the next value.
               For example a setpoint should have period 0, which will lead to a stepped
               line, whereas for a measured value like a temperature, period should be
-              slightly bigger than the poll interval. In order to make full use of this,
+              equal to the poll interval. In order to make full use of this,
               we would need some additional parameter property.
           - show: True/False, whether this curve should be shown or not by default in
               a summary chart

secop_psi/mercury.py

@@ -154,7 +154,7 @@ class HasProgressCheck:
     def check_progress(self, value, target):
         """called from read_status
 
-        indented to be also be used for alterative implementations of read_status
+        intended to be also be used for alternative implementations of read_status
         """
         base = max(abs(target), abs(value))
         tol = base * self.relative_tolerance + self.tolerance