--dry-run and --diff command-line options

It's already a work in progress (!1573 (merged), !1563 (merged), !1536 (merged)); let's track here the possible improvements or discussion topics:

• description text of the --dry-run option, !1564 (242d4e25)
• narrative documentation about these options, !1564 (comment 2364240093)
• information about the dry-run mode being used in command output, !1577 (merged)
• discuss about --diff
  • should we continue exposing both =unified and =ansible values for this option?
  • or only keep --diff for the unified format and add an --ansible-diff option?
  • then make it non-experimental and unhide it