[css-properties-values-api] Clarify computed value of <transform-function>. (#782)

The current text doesn't actually mention what should happen with
`<transform-function>` values that do NOT appear in `<transform-list>`.

Author: andruud

css-properties-values-api/Overview.bs

@@ -424,7 +424,7 @@ computed value is as specified. Calc expressions that are
 <number> and <percentage> values get reduced during computation to simple
 numbers and percentages respectively.
 
-For <transform-function> values contained in <transform-list> values,
+For <transform-function> values (including those contained in <transform-list> values),
 the computed value is as specified but with all lengths resolved to their
 computed values.