parser.yy is actually quite explicit here:
// We must not have lookahead tokens parsed in lyric mode. In order
// to save confusion, we take almost the same set as permitted with
// \lyricmode and/or \lyrics. However, music identifiers are also
// allowed, and they obviously do not require switching into lyrics
// mode for parsing.
lyric_mode_music:
{
parser->lexer_->push_lyric_state ();
} grouped_music_list
{
parser->lexer_->pop_state ();
$$ = $2;
}
| MUSIC_IDENTIFIER
;
It's not like LilyPond is overdocumented. When there is an explicit
comment, chances are that it's sort-of relevant.