add comment clarifying the logic behind the chosen numpy versions

python/requirements-wheel-test.txt

@@ -5,6 +5,10 @@ pytest
 pytz
 tzdata; sys_platform == 'win32'
 
+# We generally test with the oldest numpy version that supports a given Python
+# version. However, there is no need to make this strictly the oldest version,
+# so it can be broadened to have a single version specification across platforms.
+# (`~=x.y.z` specifies a compatible release as `>=x.y.z, == x.y.*`)
 numpy~=1.21.3; python_version < "3.11"
 numpy~=1.23.2; python_version == "3.11"
 numpy~=1.26.0; python_version == "3.12"