genrest: Show symbol help texts on index page

Instead of showing the prompt for symbols on the index page, show their
help texts. This makes searching easier.

Fall back on the prompt if no help text is available.

Also change the code to only show one of the prompts if several are
available (happens if a symbol is defined in multiple locations and adds
a prompt in more than one of them). It's probably overkill to show them
all, and it doesn't come up that often.

Suggested by David B. Kinder.

Co-authored-by: David B. Kinder <david.b.kinder@intel.com>
Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>