Update geo_shape docs to include unsupported features (#38138)

There are a two major features that are not yet supported by BKD Backed geo_shape: MultiPoint queries, and CONTAINS relation. It is important we are explicitly clear in the documentation that using the new approach may not work for users that depend on these features. This commit adds an IMPORTANT NOTE section to geo_shape docs that explicitly highlights these missing features and what should be done if they are an absolute necessity.

Author: nknize

docs/reference/mapping/types/geo-shape.asciidoc

@@ -21,7 +21,7 @@ type.
 |=======================================================================
 |Option |Description| Default
 
-|`tree |deprecated[6.6, PrefixTrees no longer used] Name of the PrefixTree
+|`tree` |deprecated[6.6, PrefixTrees no longer used] Name of the PrefixTree
 implementation to be used: `geohash` for GeohashPrefixTree and `quadtree`
 for QuadPrefixTree. Note: This parameter is only relevant for `term` and
 `recursive` strategies.
@@ -127,6 +127,20 @@ the `tree` or `strategy` parameters according to the appropriate
 <<geo-shape-mapping-options>>. Note that these parameters are now deprecated
 and will be removed in a future version.
 
+*IMPORTANT NOTES*
+
+The following features are not yet supported with the new indexing approach:
+
+* `geo_shape` query with `MultiPoint` geometry types - Elasticsearch currently prevents searching
+   geo_shape fields with a MultiPoint geometry type to avoid a brute force linear search
+   over each individual point. For now, if this is absolutely needed, this can be achieved
+   using a `bool` query with each individual point.
+
+* `CONTAINS` relation query - when using the new default vector indexing strategy, `geo_shape`
+   queries with `relation` defined as `contains` are not yet supported. If this query relation
+   is an absolute necessity, it is recommended to set `strategy` to `quadtree` and use the
+   deprecated PrefixTree strategy indexing approach.
+
 [[prefix-trees]]
 [float]
 ==== Prefix trees