Update missing data handling APIs in docs (rapidsai#11345)

This PR updates a few missing data operation APIs in docs.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: rapidsai#11345

docs/cudf/source/api_docs/dataframe.rst

@@ -187,14 +187,18 @@ Missing data handling
 .. autosummary::
    :toctree: api/
 
+   DataFrame.backfill
+   DataFrame.bfill
    DataFrame.dropna
+   DataFrame.ffill
    DataFrame.fillna
    DataFrame.interpolate
    DataFrame.isna
    DataFrame.isnull
    DataFrame.nans_to_nulls
    DataFrame.notna
    DataFrame.notnull
+   DataFrame.pad
    DataFrame.replace
 
 Reshaping, sorting, transposing

docs/cudf/source/api_docs/series.rst

@@ -185,13 +185,17 @@ Missing data handling
 .. autosummary::
    :toctree: api/
 
+   Series.backfill
+   Series.bfill
    Series.dropna
+   Series.ffill
    Series.fillna
    Series.isna
    Series.isnull
    Series.nans_to_nulls
    Series.notna
    Series.notnull
+   Series.pad
    Series.replace
 
 Reshaping, sorting
@@ -200,11 +204,9 @@ Reshaping, sorting
    :toctree: api/
 
    Series.argsort
-   Series.interleave_columns
    Series.sort_values
    Series.sort_index
    Series.explode
-   Series.scatter_by_map
    Series.searchsorted
    Series.repeat
    Series.transpose