[DOCS] Clarify that "now" cannot be used in `date_range` at index time (#52446)

`date_range` fields do not accept `"now"` as a value of either bounds at indexing time.

This corrects an error in the range data type mapping docs.

docs/reference/mapping/types/range.asciidoc

@@ -54,7 +54,7 @@ PUT range_index/_doc/1?refresh
 <1> `date_range` types accept the same field parameters defined by the <<date, `date`>> type.
 <2> Example indexing a meeting with 10 to 20 attendees.
 <3> Date ranges accept the same format as described in <<ranges-on-dates, date range queries>>.
-<4> Example date range using date time stamp. This also accepts <<date-math, date math>> formatting, or "now" for system time.
+<4> Example date range using date time stamp. This also accepts <<date-math, date math>> formatting. Note that "now" cannot be used at indexing time.
 
 The following is an example of a <<query-dsl-term-query, term query>> on the `integer_range` field named "expected_attendees".