Fix type predicates/docstrings for two music properties.

Fix type predicates/docstrings for two music properties.

* scm/define-music-properties.scm (all-music-properties):

  fix type predicates for 'span-type and 'span-text

  emend decscriptions, since both properties are now only used for dynamics

* scm/define-music-types.scm (music-descriptions):

  remove 'span-type setting for LigatureEvent