Remove asciidocs

This adds a sentence to `redirects.asciidoc` explaining what frozen
indices were - otherwise, everything will point to the message about
the unfreeze API having gone away, which is not very helpful. Some
cross-references are updated to point to this rather than to the
notice about the removal of the unfreeze API.

Author: PeteGillinElastic

docs/reference/indices.asciidoc

@@ -24,7 +24,6 @@ index settings, aliases, mappings, and index templates.
 * <<indices-split-index>>
 * <<indices-clone-index>>
 * <<indices-rollover-index>>
-* <<unfreeze-index-api>>
 * <<indices-resolve-index-api>>
 * <<indices-resolve-cluster-api>>
 * <<indices-downsample-data-stream>>
@@ -143,6 +142,5 @@ include::indices/shrink-index.asciidoc[]
 include::indices/simulate-index.asciidoc[]
 include::indices/simulate-template.asciidoc[]
 include::indices/split-index.asciidoc[]
-include::indices/apis/unfreeze.asciidoc[]
 include::indices/update-settings.asciidoc[]
 include::indices/put-mapping.asciidoc[]

docs/reference/indices/apis/unfreeze.asciidoc

@@ -43,7 +43,7 @@ For more information on managing indices, refer to <<indices, Index APIs>>.
 
 * To filter the list of indices, use the search bar or click a badge.
 Badges indicate if an index is a <<ccr-put-follow,follower index>>, a
-<<rollup-get-rollup-index-caps,rollup index>>, or <<unfreeze-index-api,frozen>>.
+<<rollup-get-rollup-index-caps,rollup index>>, or <<frozen-indices,frozen>>.
 
 * To drill down into the index
 <<mapping,mappings>>, <<index-modules-settings,settings>>, and statistics,

docs/reference/indices/index-mgmt.asciidoc

@@ -156,10 +156,16 @@ See <<restore-entire-cluster>>.
 The freeze index API was removed in 8.0.
 // tag::frozen-removal-explanation[]
 Frozen indices are no longer useful due to
-https://www.elastic.co/blog/significantly-decrease-your-elasticsearch-heap-memory-usage[recent
-improvements in heap memory usage].
+https://www.elastic.co/blog/significantly-decrease-your-elasticsearch-heap-memory-usage[improvements
+in heap memory usage].
 // end::frozen-removal-explanation[]
 
+[role="exclude",id="unfreeze-index-api"]
+=== Unfreeze index API
+
+The unfreeze index API was removed in 9.0.
+include::redirects.asciidoc[tag=frozen-removal-explanation]
+
 [role="exclude",id="ilm-freeze"]
 === Freeze {ilm-init} action
 
@@ -1749,8 +1755,10 @@ See <<search-terms-enum>>.
 === Frozen indices
 
 // tag::frozen-index-redirect[]
-
-For API documentation, see <<unfreeze-index-api>>.
+Older versions of {es} provided the option to reduce the amount of data kept in memory for an index, at the expense of
+increasing search latency. This was known as 'freezing' the index.
+include::redirects.asciidoc[tag=frozen-removal-explanation]
+The freeze index API was removed in 8.0, and the unfreeze index API was removed in 9.0.
 // end::frozen-index-redirect[]
 
 [role="exclude",id="best_practices"]

docs/reference/redirects.asciidoc

@@ -100,7 +100,7 @@ requires the keyword `LIKE` for SQL `LIKE` pattern.
 [[sql-index-frozen]]
 === Frozen Indices
 
-By default, {es-sql} doesn't search <<unfreeze-index-api,frozen indices>>. To
+By default, {es-sql} doesn't search <<frozen-indices,frozen indices>>. To
 search frozen indices, use one of the following features:
 
 dedicated configuration parameter::