Clarify that exemplar reservoir default may change in a minor version (open-telemetry#3943)

Related to open-telemetry#3756. 

See this
[conversation](open-telemetry#3870 (comment))
for context.

---------

Co-authored-by: Reiley Yang <reyang@microsoft.com>

Author: jack-berg

CHANGELOG.md

@@ -13,6 +13,9 @@ release.
 
 ### Metrics
 
+- Clarify that exemplar reservoir default may change in a minor version.
+  ([#3943](https://github.com/open-telemetry/opentelemetry-specification/pull/3943))
+
 ### Logs
 
 - Refine description of Instrumentation Scope.

specification/metrics/sdk.md

@@ -1052,6 +1052,11 @@ use `AlignedHistogramBucketExemplarReservoir`.
   twenty (e.g. `min(20, max_buckets)`).
 - All other aggregations will use `SimpleFixedSizeExemplarReservoir`.
 
+Exemplar default reservoirs MAY change in
+a [minor version bump](./../versioning-and-stability.md#minor-version-bump). No
+guarantees are made on the shape or statistical properties of returned
+exemplars.
+
 #### SimpleFixedSizeExemplarReservoir
 
 This reservoir MUST use an uniformly-weighted sampling algorithm based on the