Enhance the claim of auto random type limitation

[BornChanger]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

2. Existing document does not explicitly claim that R-S must be greater than 27

3. Signed type value by default generates positive value if not specified a negative base.

4. Describe your suggestion or addition.

5. Provide some reference materials (such as documents and websites) if you could.