Document all gdbus-codegen options

The --help and --xml-files options were missing from the man page.
Fix a typo
Document all glib-genmarshal options

The --stdinc option was missing from the man page.
Document all glib-mkenums placeholders

The @ENUMPREFIX@ placeholder was missing in the man page.
Document all gtester-report options

The --subunit option was missing from the man page.