[DOCS] time_series_dimension fields do not support ignore_above (elastic#106203)

* [DOCS] `time_series_dimension` fields do not support `ignore_above`

There is existing validation for this combination of parameters but
it was not documented.

Closes elastic#99044

* Remove maximum size constraint

* Add reasoning for constraints

Author: lkts

docs/reference/mapping/types/keyword.asciidoc

@@ -165,8 +165,9 @@ Dimension fields have the following constraints:
 * The `doc_values` and `index` mapping parameters must be `true`.
 * Field values cannot be an <<array,array or multi-value>>.
 // end::dimension[]
-* Field values cannot be larger than 1024 bytes.
-* The field cannot use a <<normalizer,`normalizer`>>.
+* Dimension values are used to identify a document’s time series. If dimension values are altered in any way during indexing, the document will be stored as belonging to different from intended time series. As a result there are additional constraints:
+** <<ignore-above,`ignore_above`>> mapping parameter isn't supported.
+** The field cannot use a <<normalizer,`normalizer`>>.
 --
 
 [[keyword-synthetic-source]]