Improve relevance table function documentation (#781)

* Style

* Improve description for relevance table, #762

Describe different, additional columns in relevance table for
multiclass case.

* Wording

Co-authored-by: Nils Braun <nils-braun@users.noreply.github.com>

Co-authored-by: Nils Braun <nils-braun@users.noreply.github.com>

tsfresh/feature_selection/relevance.py

@@ -146,11 +146,16 @@ def calculate_relevance_table(
 
     :return: A pandas.DataFrame with each column of the input DataFrame X as index with information on the significance
              of this particular feature. The DataFrame has the columns
-             "Feature",
+             "feature",
              "type" (binary, real or const),
              "p_value" (the significance of this feature as a p-value, lower means more significant)
              "relevant" (True if the Benjamini Hochberg procedure rejected the null hypothesis [the feature is
-             not relevant] for this feature)
+             not relevant] for this feature).
+             If the problem is `multiclass` with n classes, the DataFrame will contain n
+             columns named "p_value_CLASSID" instead of the "p_value" column.
+             `CLASSID` refers here to the different values set in `y`.
+             There will also be n columns named `relevant_CLASSID`, indicating whether
+             the feature is relevant for that class.
     :rtype: pandas.DataFrame
     """