docs(waterdata): Frame _NEVER_CHUNK as exceptions to a default-chunk rule

After investigating: the OGC getters expose ~94 list-shaped params, all
chunkable. The current 13-entry denylist captures every exception. An
allowlist would be ~7x longer and would need updating every time USGS
adds a column.

Reframe the comment to state the default rule first ("any list-shaped
kwarg gets chunked"), then enumerate the exceptions by reason
(response-shaping, structured, intervals, handled-elsewhere, scalar-by-
contract). Reads as "here are the few cases the default-chunk rule
doesn't apply" rather than "here is an arbitrary exclusion set."

Author: thodson-usgs

dataretrieval/waterdata/chunking.py

@@ -51,16 +51,21 @@
     _split_top_level_or,
 )
 
-# Params the multi-value chunker must NOT comma-join across sub-requests.
-# Categorized to make additions intentional:
-#   - response shape:  ``properties`` defines columns; sharding would
-#                       give different columns per chunk.
+# Default rule: any list-shaped kwarg with >1 element is chunked across
+# sub-requests — each chunk becomes a comma-joined sub-list in the URL.
+# The OGC getters expose ~90 such list-shaped params (IDs, codes,
+# statuses, ...), all chunkable, so it's shorter to enumerate the
+# exceptions than to maintain an allowlist that grows with the API.
+# Exceptions, by reason:
+#   - response shape: ``properties`` defines the columns; sharding
+#                      would yield different schemas per chunk.
 #   - structured:      ``bbox`` is a fixed 4-element coord tuple.
 #   - intervals:       date/time ranges are not enumerable sets.
 #   - handled elsewhere: ``filter`` gets OR-clause partitioning in
-#                         ``_plan_joint``; multi-value semantics would
-#                         emit malformed CQL.
-#   - scalar by contract: ``limit``, ``skip_geometry``, ``filter_lang``.
+#                         ``_plan_joint``; comma-joining CQL clauses
+#                         would emit malformed expressions.
+#   - scalar by contract: ``limit``, ``skip_geometry``, ``filter_lang``
+#                          — a list value would be a type-erasure smuggle.
 _NEVER_CHUNK = frozenset(
     {
         "properties",