docs: dash-dash-opt into literal or link

[pajod]

Replace all the --opt that sphinx might rewrite into –opt (typographical alternates, not accepted by argparse) by literals, or references (where applicable). Also change some https:// references into internal ones, preventing unintended navigation between doc versions.

• no functional change intended. only eases reading/copy-pasting docs
• motivated by keep-alive argument unrecognised #3325
• minor downside of this patch: parts might need to be reverted as once-references options are deprecated & removed
• can be merged independently from related style: argparse inheritance, hide some options from --help #3272

[pajod]

This is intentional. Looks weird, but only so because reStructuredText parsers are weird - no way to express a literal reference.

[benoitc]

Closing this documentation PR. Note: We're migrating away from Sphinx to MkDocs in the next version (see PR #3426), so these RST documentation changes will no longer be applicable. Thank you for the contribution.