several doc improvements

several doc improvements

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 and moduleset.rnc.
  • autotools: 'skip-install' and 'supports-static-analyzer'
  • cmake: 'skip-install' and 'cmakedir'
  • meson: 'skip-install'