Added note to ICU docs about the possible need to reindex (#21426)

when upgrading from a previous major version.

Relates #21352

docs/plugins/analysis-icu.asciidoc

@@ -7,6 +7,23 @@ libraries, including better analysis of Asian languages, Unicode
 normalization, Unicode-aware case folding, collation support, and
 transliteration.
 
+[IMPORTANT]
+.ICU analysis and backwards compatibility
+================================================
+
+From time to time, the ICU library receives updates such as adding new
+characters and emojis, and improving collation (sort) orders.  These changes
+may or may not affect search and sort orders, depending on which characters
+sets you are using.
+
+While we restrict ICU upgrades to major versions, you may find that an index
+created in the previous major version will need to be reindexed in order to
+return correct (and correctly ordered) results, and to take advantage of new
+characters.
+
+================================================
+
+
 [[analysis-icu-install]]
 [float]
 ==== Installation