[DOCS] Adds ML SSE4.2 limitation (#344)

Author: lcawl

docs/en/stack/ml/limitations.asciidoc

@@ -5,6 +5,16 @@
 The following limitations and known problems apply to the {version} release of 
 the Elastic {ml-features}:
 
+[float]
+[[ml-limitations-sse]]
+=== CPUs must support SSE4.2
+
+{ml-cap} uses Streaming SIMD Extensions (SSE) 4.2 instructions, so it works only
+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`. See
+{ref}/ml-settings.html[{ml-cap} settings in {es}].
+
 [float]
 === Categorization uses English dictionary words
 //See x-pack-elasticsearch/#3021