Hide RubyVM::Shape's interface as much as possible [ci skip]

RubyVM::Shape is usually not available (you need SHAPE_DEBUG macro,
which is not defined by default). So it seems confusing to leave
RubyVM::Shape in the document.

This hides only method definitions because, well, I can't find a way to
hide things defined by rb_define_const or rb_struct_define_under. I gave
up making the C-based documentation right. You should define things in
Ruby instead.

shape.c

@@ -555,6 +555,7 @@ rb_shape_memsize(rb_shape_t *shape)
  * Exposing Shape to Ruby via RubyVM.debug_shape
  */
 
+/* :nodoc: */
 static VALUE
 rb_shape_too_complex(VALUE self)
 {
@@ -603,6 +604,7 @@ rb_edges_to_hash(ID key, VALUE value, void *ref)
     return ID_TABLE_CONTINUE;
 }
 
+/* :nodoc: */
 static VALUE
 rb_shape_edges(VALUE self)
 {
@@ -631,6 +633,7 @@ rb_shape_edge_name(rb_shape_t * shape)
     return Qnil;
 }
 
+/* :nodoc: */
 static VALUE
 rb_shape_export_depth(VALUE self)
 {
@@ -639,6 +642,7 @@ rb_shape_export_depth(VALUE self)
     return SIZET2NUM(rb_shape_depth(shape));
 }
 
+/* :nodoc: */
 static VALUE
 rb_shape_parent(VALUE self)
 {
@@ -652,12 +656,14 @@ rb_shape_parent(VALUE self)
     }
 }
 
+/* :nodoc: */
 static VALUE
 rb_shape_debug_shape(VALUE self, VALUE obj)
 {
     return rb_shape_t_to_rb_cShape(rb_shape_get_shape(obj));
 }
 
+/* :nodoc: */
 static VALUE
 rb_shape_root_shape(VALUE self)
 {
@@ -680,6 +686,7 @@ static VALUE edges(struct rb_id_table* edges)
     return hash;
 }
 
+/* :nodoc: */
 VALUE
 rb_obj_shape(rb_shape_t* shape)
 {
@@ -699,12 +706,14 @@ rb_obj_shape(rb_shape_t* shape)
     return rb_shape;
 }
 
+/* :nodoc: */
 static VALUE
 shape_transition_tree(VALUE self)
 {
     return rb_obj_shape(rb_shape_get_root_shape());
 }
 
+/* :nodoc: */
 static VALUE
 rb_shape_find_by_id(VALUE mod, VALUE id)
 {