Improve documentation for fixed-base constants

[daira]

• Add a pointer to how the Z and U constants for fixed-base scalar multiplication can be reproduced.
• Correct errors and omissions in the documentation of the fixed-base generators.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8a2abbb) 80.64% compared to head (fbcbccf) 80.64%.

:exclamation: Current head fbcbccf differs from pull request most recent head 160a846. Consider uploading reports for the commit 160a846 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #417 +/- ## ======================================= Coverage 80.64% 80.64% ======================================= Files 30 30 Lines 3152 3152 ======================================= Hits 2542 2542 Misses 610 610 ```

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