Move some token filter migration notes (#39072)

The removal of the "nGram" and "edgeNGram" token filter was filed in a new
migrate_8_0/mappings.asciidoc document but it should better be in an
analysis.asciidoc section and file.

docs/reference/migration/migrate_8_0.asciidoc

@@ -11,6 +11,7 @@ See also <<release-highlights>> and <<es-release-notes>>.
 
 coming[8.0.0]
 
+* <<breaking_80_analysis_changes>>
 * <<breaking_80_mappings_changes>>
 
 [float]
@@ -29,4 +30,5 @@ Elasticsearch 7.x in order to be readable by Elasticsearch 8.x.
 
 =========================================
 
-include::migrate_8_0/mappings.asciidoc[]
+include::migrate_8_0/analysis.asciidoc[]
+include::migrate_8_0/mappings.asciidoc[]

docs/reference/migration/migrate_8_0/analysis.asciidoc

@@ -0,0 +1,10 @@
+[float]
+[[breaking_80_analysis_changes]]
+=== Analysis changes
+
+[float]
+==== The `nGram` and `edgeNGram` token filter names have been removed
+
+The `nGram` and `edgeNGram` token filter names that have been deprecated since
+version 6.4 have been removed. Both token filters can only be used by their 
+alternative names `ngram` and `edge_ngram` since version 7.0.

docs/reference/migration/migrate_8_0/mappings.asciidoc

@@ -2,15 +2,8 @@
 [[breaking_80_mappings_changes]]
 === Mapping changes
 
-[float]
-==== The `nGram` and `edgeNGram` token filter names have been removed
-
-The `nGram` and `edgeNGram` token filter names that have been deprecated since
-version 6.4 have been removed. Both token filters should be used by their 
-alternative names `ngram` and `edge_ngram` instead.
-
 [float]
 ==== Limiting the number of completion contexts
 
 The number of completion contexts within a single completion field
-has been limited to 10.
+has been limited to 10.