Cleanup: move public doc-strings into headers for various API's

Some doc-strings were skipped because of blank-lines between
the doc-string and the symbol and needed to be moved manually.

- Added space below non doc-string comments to make it clear
  these aren't comments for the symbols directly below them.
- Use doxy sections for some headers.

Ref T92709

source/blender/editors/mesh/meshtools.c

@@ -755,11 +755,10 @@ int ED_mesh_join_objects_exec(bContext *C, wmOperator *op)
 
 /* -------------------------------------------------------------------- */
 /** \name Join as Shapes
+ *
+ * Append selected meshes vertex locations as shapes of the active mesh.
  * \{ */
 
-/* Append selected meshes vertex locations as shapes of the active mesh,
- * return 0 if no join is made (error) and 1 of the join is done */
-
 int ED_mesh_shapes_join_objects_exec(bContext *C, wmOperator *op)
 {
   Main *bmain = CTX_data_main(C);
@@ -1197,8 +1196,7 @@ int *mesh_get_x_mirror_faces(Object *ob, BMEditMesh *em, Mesh *me_eval)
   return mirrorfaces;
 }
 
-/* selection, vertex and face */
-/* returns 0 if not found, otherwise 1 */
+/* Selection (vertex and face). */
 
 bool ED_mesh_pick_face(bContext *C, Object *ob, const int mval[2], uint dist_px, uint *r_index)
 {