[DOCS] [7.6] Admonition for Apple notarization warning (#51692)

macOS Catalina requires software to be notarized by Apple prior to
distribution. If software is not notarized, Apple may display an
'unidentified developer' warning that prevents the software from
running.

This adds a related note to our macOS installation instructions and
provides a link to workaround instructions from Apple.

docs/reference/setup/install/targz-start.asciidoc

@@ -1,5 +1,18 @@
 ==== Running Elasticsearch from the command line
 
+[IMPORTANT]
+.Unidentified developer warnings
+====
+Apple's rollout of stricter notarization requirements affected the notarization
+of the {version} {es} artifacts. If macOS Catalina displays an unidentified
+developer warning when you first run {es}, you will need to add a security
+override.
+
+To add a security override, follow the instructions in
+https://support.apple.com/guide/mac-help/open-a-mac-app-from-an-unidentified-developer-mh40616/mac[Open
+a Mac app from an unidentified developer].
+====
+
 Elasticsearch can be started from the command line as follows:
 
 [source,sh]