clarify use of horiz_only

Author: peverwhee

docs/usage/history.md

@@ -98,13 +98,17 @@ During **init** time, fields can be added to the possible field list with a call
 |:---------------------|-----------|-----------|------------------------------------------------|
 | diagnostic_name      | No        | string    | diagnostic name for the field - will be the name in netcdf output file |
 | description          | No        | string    | Description of the variable                    |
-| vdim_name            | No        | string    | vertical dimension: 'horiz_only' for no vertical dimension; 'lev' for vertical_layer_dimension; 'ilev' for vertical_interface_dimension |
+| vdim_name            | No        | string    | vertical dimension: `horiz_only` for no vertical dimension (see note below this table); 'lev' for vertical_layer_dimension; 'ilev' for vertical_interface_dimension|
 | avgflag              | No        | string    | default average flag; options: 'avg', 'lst' (instantaneous), 'min', 'max', 'var' (standard deviation) |
 | units                | No        | string    | variable units                                 |
 | gridname             | Yes       | string    | gridname on which the variable's data is mapped (defaults to the physics grid) |
 | flag_xyfill          | Yes       | string    | fill value for variable values                 |
 | mixing_ratio         | Yes       | string    | constituent mixing ratio type ('wet' or 'dry'); not set to anything if not passed in |
 
+!!!Note "horiz_only"
+    `horiz_only` is a variable, not a string. If you have a variable with only a horizontal dimension, include this use statement in your module: `use cam_history_support, only: horiz_only`
+
+
 Example:
 ```
 call history_add_field('Q', 'water vapor mixing ratio', 'lev', 'avg', 'kg kg-1', mixing_ratio='wet')