docs: Improve documentation and parsing for efi=

docs: Improve documentation and parsing for efi=

Update parse_efi_param() to use parse_boolean() for "rs", so it behaves
like other Xen booleans.

However, change "attr=uc" to not be a boolean.  "no-attr=uc" is ambiguous and
shouldn't be accepted, but accept "attr=no" as an acceptable alternative.

Update the command line documentation for consistency.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>