input: add "async" flag

Obviously, this has no effect on commands which do not support this
explicitly. A later commit will enable this for screenshots.

Also add some wording on mpv_command_async(), which has nothing to do
with this. Having a more elegant, unified behavior would be nice. But
the API function was not created for this - it's merely for running
commands _synchronously_ on the core, but without blocking the client
API caller (if the API user consistently uses only async functions).

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