ESQL: Document MV_SLICE limitations (elastic#114162)

`MV_SLICE` is useful, but loading values from lucene frequently sorts
them so `MV_SLICE` is not as useful as you think it is. It's mostly for
after, say, a `SPLIT`. This documents that and adds a link to the
section on multivalues.

It also moves similar docs to a separate paragraph in the docs for
easier reading.

Author: nik9000

docs/reference/esql/functions/description/mv_first.asciidoc

@@ -29,13 +29,18 @@
     boolean preview() default false;
 
     /**
-     * The description of the function rendered in {@code META FUNCTIONS}
-     * and the docs. These should be complete sentences.
+     * The description of the function rendered in the docs and kibana's
+     * json files that drive their IDE-like experience. These should be
+     * complete sentences but can contain asciidoc syntax. It is rendered
+     * as a single paragraph.
      */
     String description() default "";
 
     /**
-     * Detailed descriptions of the function rendered in the docs.
+     * Detailed descriptions of the function rendered in the docs. This is
+     * rendered as a single paragraph following {@link #description()} in
+     * the docs and is <strong>excluded</strong> from Kibana's IDE-like
+     * experience. It can contain asciidoc syntax.
      */
     String detailedDescription() default "";