Self-documenting fstrings detection disabled by default

[netromdk]

Some confusion has arisen since, by default, fstrings detection is enabled but fstrings self-doc detection is disabled.

This is on purpose as is pointed out in the caveats section:

Self-documenting fstrings detection has been disabled by default because the built-in AST cannot distinguish f'{a=}' from f'a={a}', for instance, since it optimizes some information away (#39). And this incorrectly marks some source code as using fstring self-doc when only using general fstring. To enable (unstable) fstring self-doc detection, use --feature fstring-self-doc.