Add "add_bounds" to api.rst

Author: dcherian

doc/api.rst

@@ -16,6 +16,7 @@ DataArray
    :toctree: generated/
    :template: autosummary/accessor_method.rst
 
+    DataArray.cf.add_bounds
     DataArray.cf.describe
     DataArray.cf.get_valid_keys
 
@@ -30,5 +31,6 @@ Dataset
    :toctree: generated/
    :template: autosummary/accessor_method.rst
 
+    Dataset.cf.add_bounds
     Dataset.cf.describe
     Dataset.cf.get_valid_keys