docstring: make RootDocstring a subclass of _CompoundDocstring

Also move RootDocstring after _CompoundDocstring.

src/hawkmoth/docstring.py

@@ -203,21 +203,6 @@ def get_name(self):
     def get_line(self):
         return self._meta['line']
 
-class RootDocstring(Docstring):
-    def __init__(self, filename=None, domain='c', clang_args=None):
-        super().__init__(domain=domain)
-        self._filename = filename
-        self._clang_args = clang_args
-
-    def get_filename(self):
-        return self._filename
-
-    def get_clang_args(self):
-        return self._clang_args
-
-    def get_domain(self):
-        return self._domain
-
 class TextDocstring(Docstring):
     _indent = 0
     _fmt = ''
@@ -271,6 +256,21 @@ def _get_decl_name(self):
 
         return self._decl_name
 
+class RootDocstring(_CompoundDocstring):
+    def __init__(self, filename=None, domain='c', clang_args=None):
+        super().__init__(domain=domain)
+        self._filename = filename
+        self._clang_args = clang_args
+
+    def get_filename(self):
+        return self._filename
+
+    def get_clang_args(self):
+        return self._clang_args
+
+    def get_domain(self):
+        return self._domain
+
 class StructDocstring(_CompoundDocstring):
     _indent = 1
     _fmt = '.. {domain}:struct:: {name}'