Tidy WQP_Metadata.site_info per /simplify review

Per /simplify review on PR #249:
- Switch `site_info` from `@property` to `functools.cached_property`.
  Repeated access of `md.site_info` (common in notebooks) now reuses
  the resolved tuple instead of issuing a fresh `what_sites` HTTP call
  on every read.
- Add a one-line comment explaining the legacy-alias -> `siteid`
  coercion: whichever of `siteid`/`sites`/`site`/`site_no` matched,
  the value is passed as `siteid` (what_sites' native WQP arg).

Also confirmed the docstring's `comments` -> `comment` rename matches
`BaseMetadata.comment` (singular) defined at `dataretrieval/utils.py:130`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/wqp.py

@@ -12,6 +12,7 @@
 from __future__ import annotations
 
 import warnings
+from functools import cached_property
 from io import StringIO
 from typing import TYPE_CHECKING
 
@@ -687,8 +688,10 @@ def __init__(
         self._legacy = legacy
         self._ssl_check = ssl_check
 
-    @property
+    @cached_property
     def site_info(self):
+        # Walk WQP-native key first, then legacy NWIS-style aliases. Whichever
+        # matched, pass the value as `siteid` -- that's what_sites' native arg.
         for key in ("siteid", "sites", "site", "site_no"):
             if key in self._parameters:
                 return what_sites(