Docs: document precision limitations of geo_bounding_box (#30540)

The geo_bounding_box query might produce false positives alongside
the right and upper edges and false negatives alongside left and
bottom edges. This commit documents the behavior and defines the
maximum error.

Closes #29196

docs/reference/query-dsl/geo-bounding-box-query.asciidoc

@@ -329,3 +329,16 @@ and will not match any documents for this query. This can be useful when
 querying multiple indexes which might have different mappings. When set to
 `false` (the default value) the query will throw an exception if the field
 is not mapped.
+
+[float]
+==== Notes on Precision
+
+Geopoints have limited precision and are always rounded down during index time.
+During the query time, upper boundaries of the bounding boxes are rounded down,
+while lower boundaries are rounded up. As a result, the points along on the
+lower bounds (bottom and left edges of the bounding box) might not make it into
+the bounding box due to the rounding error. At the same time points alongside
+the upper bounds (top and right edges) might be selected by the query even if
+they are located slightly outside the edge. The rounding error should be less
+than 4.20e-8 degrees on the latitude and less than 8.39e-8 degrees on the
+longitude, which translates to less than 1cm error even at the equator.