Update lfric_diagnostics.rst (#259)

source/WorkingPractices/Diagnostics/lfric_diagnostics.rst

@@ -14,9 +14,10 @@ adding a diagnostic amounts to just three small changes.
  For experienced or confident users adding a diagnostic to an
  existing science section, follow these steps:
 
- #. Add a new record to the
- ``lfric_atm/metadata/field_def_initial_diags.xml`` file based on
- an existing diagnostic if available.
+ #. Add a new record to the field definition file. Most diagnostic 
+ fields are defined in the
+ ``lfric_atm/metadata/field_def_diags.xml`` file. It can be based on
+ an existing diagnostic if a similar one is available.
  #. Add lines to initialise the field holding the diagnostic in the
  appropriate diagnostics initialisation module. Add the field to
  the procedure argument list. Typically, the modules are named