Fix docs for Numba UDFs. (#11020)

This PR fixes some documentation links to use rST format so they render correctly in the docs.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - https://github.com/brandon-b-miller

URL: #11020

python/cudf/cudf/core/dataframe.py

@@ -3682,12 +3682,11 @@ def apply(
         """
         Apply a function along an axis of the DataFrame.
         ``apply`` relies on Numba to JIT compile ``func``.
-        Thus the allowed operations within ``func`` are limited
-        to the ones specified
-        [here](https://numba.pydata.org/numba-doc/latest/cuda/cudapysupported.html).
-        For more information, see the cuDF guide
-        to user defined functions found
-        [here](https://docs.rapids.ai/api/cudf/stable/user_guide/guide-to-udfs.html).
+        Thus the allowed operations within ``func`` are limited to `those
+        supported by the CUDA Python Numba target
+        <https://numba.pydata.org/numba-doc/latest/cuda/cudapysupported.html>`__.
+        For more information, see the `cuDF guide to user defined functions
+        <https://docs.rapids.ai/api/cudf/stable/user_guide/guide-to-udfs.html>`__.
 
         Parameters
         ----------

python/cudf/cudf/core/series.py

@@ -2024,12 +2024,11 @@ def apply(self, func, convert_dtype=True, args=(), **kwargs):
         Similar to ``pandas.Series.apply``.
 
         ``apply`` relies on Numba to JIT compile ``func``.
-        Thus the allowed operations within ``func`` are limited
-        to the ones specified
-        [here](https://numba.pydata.org/numba-doc/latest/cuda/cudapysupported.html).
-        For more information, see the cuDF guide to
-        user defined functions found
-        [here](https://docs.rapids.ai/api/cudf/stable/user_guide/guide-to-udfs.html).
+        Thus the allowed operations within ``func`` are limited to `those
+        supported by the CUDA Python Numba target
+        <https://numba.pydata.org/numba-doc/latest/cuda/cudapysupported.html>`__.
+        For more information, see the `cuDF guide to user defined functions
+        <https://docs.rapids.ai/api/cudf/stable/user_guide/guide-to-udfs.html>`__.
 
         Parameters
         ----------