DOC: Add details to DataFrame.combine_first docstring (#46060)

* DOC: Add details to DataFrame.combine_first docstring

* fixup

Author: Moisan

pandas/core/frame.py

@@ -7448,7 +7448,11 @@ def combine_first(self, other: DataFrame) -> DataFrame:
 
         Combine two DataFrame objects by filling null values in one DataFrame
         with non-null values from other DataFrame. The row and column indexes
-        of the resulting DataFrame will be the union of the two.
+        of the resulting DataFrame will be the union of the two. The resulting
+        dataframe contains the 'first' dataframe values and overrides the
+        second one values where both first.loc[index, col] and
+        second.loc[index, col] are not missing values, upon calling
+        first.combine_first(second).
 
         Parameters
         ----------