Clarity for commands vs global options

While reviewing the `global options` section in `runtime.md`, it seemed additional clarity
was needed for the `command` and `global options` requirements.

Discussed and worked on wording with @wking via private IRC.

Also adds uppercase RFC 2119 words for this section.

With regard to the statement `Command names MUST not start with hyphens,` the rationale
behind this decision is to "distinguish unrecognized commands from unrecognized options,
and because we are "requiring runtimes to fail-fast for unrecognized commands" [1,2].

`[1]` https://github.com/wking/oci-command-line-api/pull/8/files#r46898167
`[2]` 527f3c6#commitcomment-14835617

Signed-off-by: Mike Brown <brownwm@us.ibm.com>
Reviewed-by: W. Trevor King <wking@tremily.us>

runtime.md

@@ -9,8 +9,11 @@ $ funC [global-options] <COMMAND> [command-specific-options] <command-specific-a
 
 ## Global options
 
-None are required, but the runtime may support options that start with at least one hyphen.
-Global options may take positional arguments (e.g. `--log-level debug`), but the option parsing must be such that `funC <COMMAND>` is unambiguously an invocation of `<COMMAND>` for any `<COMMAND>` that does not start with a hyphen (including commands not specified in this document).
+None are required, but the runtime MAY support options that start with at least one hyphen.
+Global options MAY take positional arguments (e.g. `--log-level debug`).
+Command names MUST not start with hyphens.
+The option parsing MUST be such that `funC <COMMAND>` is unambiguously an invocation of `<COMMAND>` (even for commands not specified in this document).
+If the runtime is invoked with an unrecognized command, it MUST exit with a nonzero exit code and MAY log a warning to stderr.
 
 ## Character encodings