Not actually related to #1905, but clarifying the pcp_combine usage s…

…tatement per dtcenter/METplus#1201 discussion.

met/docs/Users_Guide/reformat_grid.rst

@@ -74,7 +74,7 @@ Required arguments for the pcp_combine
 Optional arguments for pcp_combine
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-3. The **-field string** option defines the data to be extracted from the input files. Use this option when processing fields other than **APCP** or non-GRIB files. This option may be used multiple times and output will be created for each.
+3. The **-field string** option defines the data to be extracted from the input files. Use this option when processing fields other than **APCP** or non-GRIB files. It can be used multiple times and output will be created for each. In general, the field string should include the name and level of the requested data and be enclosed in single quotes (e.g. -field 'name="precip06h"; level="(*,*)";' for reading a NetCDF variable). It is processed as an inline configuration file and may also include data filtering, censoring, and conversion options.
 
 4. The **-name list** option is a comma-separated list of output variable names which override the default choices. If specified, the number of names must match the number of variables to be written to the output file.