[DOCS] Adding ML-specific prerequisites to setup docs (#42529)

Author: David Roberts

docs/reference/settings/ml-settings.asciidoc

@@ -8,6 +8,11 @@
 
 You do not need to configure any settings to use {ml}. It is enabled by default.
 
+IMPORTANT: {ml-cap} uses SSE4.2 instructions, so will only work on machines whose
+CPUs https://en.wikipedia.org/wiki/SSE4#Supporting_CPUs[support] SSE4.2. If you
+run {es} on older hardware you must disable {ml} (by setting `xpack.ml.enabled`
+to `false`).
+
 All of these settings can be added to the `elasticsearch.yml` configuration file. 
 The dynamic settings can also be updated across a cluster with the 
 <<cluster-update-settings,cluster update settings API>>.

docs/reference/setup/install/windows.asciidoc

@@ -12,6 +12,14 @@ You can continue using the `.zip` approach if you prefer.
 
 include::license.asciidoc[]
 
+NOTE: On Windows the Elasticsearch {ml} feature requires the Microsoft Universal
+C Runtime library. This is built into Windows 10, Windows Server 2016 and more
+recent versions of Windows. For older versions of Windows it can be installed
+via Windows Update, or from a
+https://support.microsoft.com/en-us/help/2999226/update-for-universal-c-runtime-in-windows[separate download].
+If you cannot install the Microsoft Universal C Runtime library you can still
+use the rest of Elasticsearch if you disable the {ml} feature.
+
 The latest stable version of Elasticsearch can be found on the
 link:/downloads/elasticsearch[Download Elasticsearch] page.
 Other versions can be found on the

docs/reference/setup/install/zip-windows.asciidoc

@@ -11,6 +11,14 @@ experience for Windows. You can continue using the `.zip` approach if you prefer
 
 include::license.asciidoc[]
 
+NOTE: On Windows the Elasticsearch {ml} feature requires the Microsoft Universal
+C Runtime library. This is built into Windows 10, Windows Server 2016 and more
+recent versions of Windows. For older versions of Windows it can be installed
+via Windows Update, or from a
+https://support.microsoft.com/en-us/help/2999226/update-for-universal-c-runtime-in-windows[separate download].
+If you cannot install the Microsoft Universal C Runtime library you can still
+use the rest of Elasticsearch if you disable the {ml} feature.
+
 The latest stable version of Elasticsearch can be found on the
 link:/downloads/elasticsearch[Download Elasticsearch] page.
 Other versions can be found on the