Add a rule to clarify vertical stagger suffixes

StandardNamesRules.rst

@@ -53,9 +53,22 @@ CCPP Standard Name Rules
 
 #. By default (when not specified otherwise), variables are grid means or centers
    (defined by the host). If a variable is defined at a different physical location,
-   a qualifier should be used to denote this. For example, for variables
-   representing quantities at the interface between grid cells vertically,
-   use at_interface.
+   a qualifier should be used to denote this. For example, to specify the vertical
+   location of a variable with respect to vertical grid cells, the following rules
+   apply. If ``[variable]`` doesn't have a vertical location suffix and is defined
+   on `n` levels:
+
+   * ``[variable]_at_interface`` is defined at the interfaces between grid cells
+     vertically, including the bottom-most and top-most interfaces. It is defined on
+     `n+1` levels.
+
+   * ``[variable]_at_top_interfaces`` is defined at the interfaces between grid
+     cells vertically, including the top-most interface *but excluding the
+     bottom-most interface*. It is defined on `n` levels.
+
+   * ``[variable]_at_bottom_interfaces`` is defined at the interfaces between grid
+     cells vertically, including the bottom-most interface *but excluding the
+     top-most interface*. It is defined on `n` levels.
 
 #. By default, *mixing_ratio* refers to mass mixing ratios. The long name should
    explicitly specify that it refers to the *mass* mixing ratio.