Thank you for your reply. I looked for the source code for the documentation but just found the source for the Arduino IDE and firmware. I hope to be able to submit useful pull requests to that project on GitHub too, but for now I was hoping to contribute to accurately describing the limitations of the library in its documentation.
The Arduino reference home says at the bottom of the page, "Corrections, suggestions, and new documentation should be posted to the Forum." (with a link here).
I suggest adding the following bullet point in the "Limitations" section of the SoftwareSerial page
- Data bits, parity, and stop bits must be 8-N-1