Document XRSPATIAL_GEOTIFF_STRICT env var in geotiff reference (#1662)

Author: brendancol

docs/source/reference/geotiff.rst

@@ -20,3 +20,24 @@ Writing
     xrspatial.geotiff.to_geotiff
     xrspatial.geotiff.write_geotiff_gpu
     xrspatial.geotiff.write_vrt
+
+Strict mode (``XRSPATIAL_GEOTIFF_STRICT``)
+==========================================
+
+Several internal helpers historically returned ``None`` when something went
+wrong: pyproj failing to parse a WKT string, a VRT source file being
+missing, a GPU helper (GDS, nvCOMP, nvJPEG, nvJPEG2000) hitting a CUDA or
+library error. These now emit :class:`xrspatial.geotiff.GeoTIFFFallbackWarning`
+with the original exception type and message.
+
+Set ``XRSPATIAL_GEOTIFF_STRICT=1`` (or ``true``, ``yes``) to promote those
+warnings into raised exceptions. The same env var also forces
+``read_geotiff_gpu(on_gpu_failure='auto')`` to behave like
+``on_gpu_failure='strict'`` so CI can fail loudly when the GPU fast path
+silently falls back to CPU.
+
+.. code-block:: bash
+
+    XRSPATIAL_GEOTIFF_STRICT=1 pytest xrspatial/geotiff/tests/
+
+See issue #1662 for the audit and the full list of affected call sites.