Update to the documentation explaining SalesForce API Name and WebForm Field Key

I think it is important to let users know that the SalesForce API Name is stored in the WebForm Field Key. The SalesForceField Key drop-down populates the WebForm Field Key with the SalesForce API Name, this maps the SalesForce Fields to the Web Form Fields.

The reason the INSTALL or web documentation should be updated is because this field is editable by the user. So if a user selects a SalesForceField Key drop-down, which populates the Field Key, and then changes the Field Key, the map can become no longer valid.

I would say make the Field Key read-only if the use SalesForce is selected for the webform, but I am not sure if there is a use case where it is helpful to be able to edit the Field Key in certain situations to allow certain configuration to work.

So at least describing this in the documentation should minimize user's having issues along these lines.