[doc] Document OTP field configuration options

Currently the Direct Access Memory Map in the [OTP documentation](https://docs.opentitan.org/hw/ip/otp_ctrl/doc/) lists the SW configuration fields but does not specify:

1) What the purpose is (beyond what can be inferred from the name of the field).
2) How the field is laid out (some large fields are arrays).
3) What the valid configuration field values are.

I've picked values for some configuration options in #8330. It would be good to document them somewhere other than header files.

Start with the Direct Access Memory Map in the linked OTP documentation, then compare the configuration choices from #8330 with the header files. Document each SW configuration field's purpose, layout, and valid values; the work is done when these details are available outside the headers.

Written by the indexing model from the issue text.