Describe behavior with synthetic source keep arrays and ignored and malformed values.

Backporting elastic#143230 to 8.19 branch.

Author: martijnvg

docs/reference/mapping/fields/synthetic-source.asciidoc

@@ -218,6 +218,8 @@ This is controlled through param `synthetic_source_keep` with the following opti
 The synthetic source fragment for such arrays is not guaranteed to match the original source exactly, e.g. array
 `[1, 2, [5], [[4, [3]]], 5]` may appear as-is or in an equivalent format like `[1, 2, 5, 4, 3, 5]`. The exact format
 may change in the future, in an effort to reduce the storage overhead of this option.
+Additionally, if `index.mapping.ignore_above` or `index.mapping.ignore_malformed` index settings are enabled then
+malformed or ignored elements are always appended last in unspecified order.
 - `all`: the source for both singleton instances and arrays of the corresponding field or object gets recorded. When
 applied to objects, the source of all sub-objects and sub-fields gets captured. Furthermore, the original source of
 arrays gets captured and appears in synthetic source with no modifications.