Improve directions for Feeds Mapping when using this module

Problem/Motivation

I did just spend a few hours looking to understand what exactly is meant with the provided direction: "Enter the exact Xlsx column name. This is case-sensitive."

Steps to reproduce

Any feed using this module:

1. Go to Mapping page
2. For any mapping pair, select the dropdown under the source - select "New XLSX column source"
3. Under the dropdown, the empty column awaits a value with the above help text. It is not clear what "column name" the module is expecting. Is it the column's letter? A full Row+Column spec? If so, does it want an implicit "1" appended (or "2" if there is a header row)? Only after debugging XlsxParser.php did I see that the incoming record "key" IS the . It presumes a tabular format with a header row, the header values in Row 1 are unique for each column. If spreadsheet's C1 value is "Foobar", then "Foobar" is the key for the inbound record and is what should be typed in the mapping source field: "Foobar".

Proposed resolution

Add the above copy to the helptext of the Source field - perhaps this: "Enter the exact value that is in Row 1 for the spreadsheet Column you wish to import from Row 2 and thereafter. The exact value is case sensitive and includes all non-alphanumeric values therein. If your spreadsheet does not have a Header row, simply indicate the number of the column starting with '0' for Column 'A'."

Remaining tasks

None.

User interface changes

See above.

API changes

None.

Data model changes

None.