[DOC] Document environment variable for failing on fallback in `cudf.…

…pandas` (#16932)

This PR documents the environment variable `CUDF_PANDAS_FAIL_ON_FALLBACK` which causes `cudf.pandas` to raise an error when fallback occurs.

Authors:
  - Matthew Murray (https://github.com/Matt711)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #16932

docs/cudf/source/developer_guide/cudf_pandas.md

@@ -136,3 +136,21 @@ Arrays are not almost equal to 7 decimals
  ACTUAL: 1.0
  DESIRED: 2.0.
 ```
+
+Setting the environment variable `CUDF_PANDAS_FAIL_ON_FALLBACK` causes `cudf.pandas` to fail when falling back from cuDF to Pandas.
+For example,
+```python
+import cudf.pandas
+cudf.pandas.install()
+import pandas as pd
+import numpy as np
+
+df = pd.DataFrame({
+ 'complex_col': [1 + 2j, 3 + 4j, 5 + 6j]
+})
+
+print(df)
+```
+```
+ProxyFallbackError: The operation failed with cuDF, the reason was <class 'NotImplementedError'>: Series with Complex128DType is not supported.
+```

python/cudf/cudf/pandas/fast_slow_proxy.py

@@ -965,7 +965,7 @@ def _fast_slow_function_call(
  except Exception as err:
  if _env_get_bool("CUDF_PANDAS_FAIL_ON_FALLBACK", False):
  raise ProxyFallbackError(
- f"The operation failed with cuDF, the reason was {type(err)}: {err}."
+ f"The operation failed with cuDF, the reason was {type(err)}: {err}"
  ) from err
  with nvtx.annotate(
  "EXECUTE_SLOW",