[luigi617]

Issue #, if available:

Description of changes:
Validate wait sub-commands and unknown options in example commands and fix existing violations

CommandValidator stopped at the operation and ignored leftover arguments, so aws ec2 wait no-such-waiter and unknown options such as --bogus-flag passed.

• CommandValidator in tests/functional/docs/test_examples.py now follows sub-commands as the CLI does, so waiter names and waiter arguments are checked
• Leftover arguments are reported as unknown options, and wait with no waiter name is reported
• --cli-input-* is handled for waiters too, Windows ^ and ` continuations are parsed, and blocks with several commands are split
• Fixed 36 example files, e.g. wrong option names, options the operation doesn't have, a wrong command in iot/add-thing-to-billing-group.rst, and placeholders in healthlake
• Moved the task example from ecs/wait/services-stable.rst into a new ecs/wait/tasks-running.rst

Testing

• Run pytest tests/functional/docs/test_examples.py -n auto
  === 64319 passed in 27.37s =====
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