parse-options: PARSE_OPT_KEEP_UNKNOWN only applies to --options

parse-options: PARSE_OPT_KEEP_UNKNOWN only applies to --options

The description of 'PARSE_OPT_KEEP_UNKNOWN' starts with "Keep unknown
arguments instead of erroring out".  This is a bit misleading, as this
flag only applies to unknown --options, while non-option arguments are
kept even without this flag.

Update the description to clarify this, and rename the flag to
PARSE_OPTIONS_KEEP_UNKNOWN_OPT to make this obvious just by looking at
the flag name.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>