This enables clap's [wrap-help] feature and sets max_term_width to wrap
after 100 columns (the value clap is planning to default to in clap-v5).

This commit also adds blank lines which cause clap to split longer doc
comments into separate help (displayed for `-h`) and long_help
(displayed for `--help`) messages, as per [doc-processing].

[wrap-help]:
https://docs.rs/clap/4.5.49/clap/_features/index.html#optional-features
[doc-processing]:
https://docs.rs/clap/4.5.49/clap/_derive/index.html#pre-processing

![before: some lines of help text stretch across the whole screen.
after: all lines are wrapped at 100 columns, and some manual linebreaks
are preserved where it makes sense (in particular, when listing the
user-data-dir locations on each
platform)](https://github.com/user-attachments/assets/359067b4-5ffb-4fe3-80bd-5e1062986417)


Release Notes:

- N/A