docs(waterdata): keep unbounded-cap rationale only at the code site

Follow-up to the prior commit, which trimmed the test but lost the
chunking.py half of the de-dup (a stray `git checkout` during local
verification reverted it). The "None lifts the cap / semaphore sized at
plan.total that can never block" mechanism is now stated once — at the
inline comment above the `semaphore = ...` line — while the _run
docstring body and the max_concurrent param doc state just the
user-facing effect.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/chunking.py

@@ -1554,8 +1554,7 @@ async def _run(self, max_concurrent: int | None) -> tuple[pd.DataFrame, Any]:
 
         The gather dispatches *every* pending sub-request, but an
         ``asyncio.Semaphore`` gates the fetches so at most
-        ``N = max_concurrent`` are in flight (``None`` degenerates to a
-        cap at the plan total — a gate that can never block);
+        ``N = max_concurrent`` are in flight (``None`` lifts the cap);
         ``N=1`` is just a sequential gather (one request at a time) and
         ``total <= 1`` is just a one-element gather. The client's
         connection pool is sized to match
@@ -1587,8 +1586,7 @@ async def _run(self, max_concurrent: int | None) -> tuple[pd.DataFrame, Any]:
         ----------
         max_concurrent : int or None
             Maximum sub-requests in flight (the semaphore value, and the
-            connection-pool size). ``None`` lifts the cap: the gate
-            degenerates to the plan total and the pool goes unbounded.
+            connection-pool size). ``None`` lifts the cap entirely.
 
         Returns
         -------