DOC: is confusing for ddof parameter of sem, var and std functions (pandas-dev#19986)

Author: alinde1

pandas/core/generic.py

@@ -7766,7 +7766,8 @@ def _doc_parms(cls):
     If the axis is a MultiIndex (hierarchical), count along a
     particular level, collapsing into a %(name1)s
 ddof : int, default 1
-    degrees of freedom
+    Delta Degrees of Freedom. The divisor used in calculations is N - ddof,
+    where N represents the number of elements.
 numeric_only : boolean, default None
     Include only float, int, boolean columns. If None, will attempt to use
     everything, then use only numeric data. Not implemented for Series.