There are 3 places where mirroring is documented:
- https://www.blender.org/manual/modeling/meshes/editing/basics/mirror.html
- https://www.blender.org/manual/modeling/meshes/editing/deforming/mirror.html
- https://www.blender.org/manual/modeling/meshes/editing/duplicating/mirror.html
These need to be consolidated and be placed into correct part of the manual.