More clearly document what INPUT_FILE_PATH can be

[jfrost-mo]

What problem does your feature request solve?

• It is unclear from the documentation if this is a directory (as in the RES) or if this includes the files as well.
• This is further complicated by the CSET_FILE_NAME_METADATA_PATTERN under CSET_INCREMENTAL_DATA_FETCH
• This may be something that will become clearer when multiple dates can be added as I imagine it will use the naming conventions that people are used to when cycling over different forecast cycles.
• Related: Having a directory in the INPUT_FILE_PATH breaks #549

[jfrost-mo]

Fixed in #765.