Bug 31984: TaxRate preference - add note about vendor updates

Bug 31984: TaxRate preference - add note about vendor updates

If a value for the TaxRates system preference is changed or
removed, vendors retain the original value recorded in their
tax rate field.

The vendor tax rate is used to calculate prices for orders
added to a basket. Where the tax rate doesn't match with the
system preference, a warning message is displayed on the order
form "Tax rate not defined in system preference TaxRates!".

This patch adds a note to the system preference description
about updating vendor tax rate information where required.

Test plan:
1. Check the values recorded for the TaxRates system preference.
2. For a vendor (Acquisitions > search for a vendor > Edit vendor),
   check the value recorded in the tax rate field.
3. Note that the tax rate field is populated from a dropdown list
   using values from the TaxRates system preference.
4. Change the TaxRates system preferemce to remove a tax
   rate. For example, change 0|0.12|0.1965 to 0|0.12.
5. Refresh the vendor page.
6. Note that the vendor tax rate remains as the original value
   until you edit the vendor.
7. Apply the patch.
8. Review the description for the TaxRates system preference and
   make sure the new note about updating vendor tax rates where
   required makes sense.
9. Sign off!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>