services/horizon/flags: refined cli help content

[sreuland]

What problem does your feature solve?

current help output printed to console from stellar-horizon -h and sub-commands likestellar-horizon <sub-command> -h prints all Global Flags in any case, many of which are not relevant within specific sub-commands.

the Global Flags section can be noisy, making difficult to understand what parameters are relevant for each specific sub-command like serve, db, ingest, record-metrics

What would you like to see?

reduced cli help output, less copy, but what is printed is more concise to context.

• stellar-horizon -h doesn't print Global Flags, just the sub-commands available and new 'best practices' blurb to recommend using environment variables overall.

• stellar-horizon <sub-command> ... -h, doesn't print Global Flags, just the flags relevant for the <sub-command>'s given, i.e. the help output for stellar-horizon db init vs stellar-horizon db reingest

What alternatives are there?

[sreuland]

related to: #4915