docs(waterdata): drop R-analogue notes from getter docstrings

The combined-metadata, field-measurements-metadata, and ratings
docstrings (and the ratings module docstring) each pointed at their R
dataRetrieval counterpart. Those cross-package references don't help a
Python caller reading help(...), and the other getters don't carry them,
so remove them and let each docstring describe the function on its own
terms. The OpenAPI/service reference links and the Python-to-Python
"analogue to get_time_series_metadata" cross-reference are kept.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/api.py

@@ -1047,8 +1047,6 @@ def get_combined_metadata(
 
     See the OpenAPI reference for the full list of supported fields:
     https://api.waterdata.usgs.gov/ogcapi/v0/openapi?f=html#/combined-metadata
-    The R analogue is ``read_waterdata_combined_meta`` in
-    https://github.com/DOI-USGS/dataRetrieval/.
 
     All ~35 location-catalog kwargs are accepted (``agency_code``,
     ``state_name``, ``drainage_area``, ``aquifer_code``, …) but only
@@ -1813,8 +1811,6 @@ def get_field_measurements_metadata(
 
     See the OpenAPI reference for the full list of supported fields:
     https://api.waterdata.usgs.gov/ogcapi/v0/openapi?f=html#/field-measurements-metadata
-    The R analogue is ``read_waterdata_field_meta`` in
-    https://github.com/DOI-USGS/dataRetrieval/.
 
     Parameters
     ----------

dataretrieval/waterdata/ratings.py

@@ -5,9 +5,6 @@
 files (``exsa``, ``base``, ``corr``) for active streamgages — see the
 service overview at https://api.waterdata.usgs.gov/docs/stac/ and the
 WDFN announcement at https://waterdata.usgs.gov/blog/wdfn-rating-curves/.
-
-The R analogue is ``read_waterdata_ratings`` in
-https://github.com/DOI-USGS/dataRetrieval/.
 """
 
 from __future__ import annotations
@@ -65,8 +62,7 @@ def get_ratings(
 
     See https://api.waterdata.usgs.gov/docs/stac/ for the upstream service
     docs and https://waterdata.usgs.gov/blog/wdfn-rating-curves/ for the
-    background announcement. The R analogue is ``read_waterdata_ratings``
-    in https://github.com/DOI-USGS/dataRetrieval/.
+    background announcement.
 
     Parameters
     ----------