Clarify "precision" in currency format functions

The documentation of the FormatAccounting and FormatCurrency
functions could be clearer in terms of how the precision param
works. This commit makes it more explicit that adding a precision
of < 2 will not format the return values to include fewer decimals.

Resolves gohugoio#8858

tpl/lang/lang.go

@@ -80,8 +80,10 @@ func (ns *Namespace) FormatPercent(precision, number interface{}) (string, error
  return ns.translator.FmtPercent(n, p), nil
 }
 
-// FormatCurrency returns the currency reprecentation of number for the given currency and precision
+// FormatCurrency returns the currency representation of number for the given currency and precision
 // for the current language.
+//
+// The return value is formatted with at least two decimal places.
 func (ns *Namespace) FormatCurrency(precision, currency, number interface{}) (string, error) {
  p, n, err := ns.castPrecisionNumber(precision, number)
  if err != nil {
@@ -94,8 +96,10 @@ func (ns *Namespace) FormatCurrency(precision, currency, number interface{}) (st
  return ns.translator.FmtCurrency(n, p, c), nil
 }
 
-// FormatAccounting returns the currency reprecentation of number for the given currency and precision
+// FormatAccounting returns the currency representation of number for the given currency and precision
 // for the current language in accounting notation.
+//
+// The return value is formatted with at least two decimal places.
 func (ns *Namespace) FormatAccounting(precision, currency, number interface{}) (string, error) {
  p, n, err := ns.castPrecisionNumber(precision, number)
  if err != nil {