moduleset-syntax.rst:
 - Document the automatic expansion of ${prefix} and ${libdir}
   in command args attributes.
 - Document that 'makeinstallargs' attribute must include the
   make target.
 - Document default values for skip-install.
 - Document some attributes that take a yes/no instead of true/false,
   also mention their default values.
 - Document the supports-parallel-builds attribute.
 - Add some missing attributes to their module.
 - Highlight some terms.

moduleset.dtd and moduleset.rnc:
 - Add some attributes that were documented in moduleset-syntax.rst
   but were not added to moduleset.{dtd|rnc}.
      autotools: skip-install and supports-static-analyzer
      cmake: skip-install and cmakedir
      meson: skip-install