docs: Document use of NONE in expected_region_highlight

tests/README.md

@@ -12,6 +12,8 @@ The value of that parameter is a list of strings of the form  `"$i $j $style"`.
 or `"$i $j $style $todo"`.
 Each string specifies the highlighting that `$BUFFER[$i,$j]` should have;
 that is, `$i` and `$j` specify a range, 1-indexed, inclusive of both endpoints.
+`$style` is either a key of `$ZSH_HIGHLIGHT_STYLES` or `NONE` to specify no
+highlighting should be observed.
 If `$todo` exists, the test point is marked as TODO (the failure of that test
 point will not fail the test), and `$todo` is used as the explanation.