[DOCS] Reflect recent improvements in notes on watch/datafeed privileges (elastic/x-pack-elasticsearch#4116)

Following elastic/x-pack-elasticsearch#3254 security for ML datafeeds has been improved.  The same goes
for watches since elastic/x-pack-elasticsearch#2808.

This change updates a section of the docs that was missed in those changes.
(The majority of the docs changes were made at the appropriate time.)

Original commit: elastic/x-pack-elasticsearch@b3b24ca483

docs/en/security/reference/privileges.asciidoc

@@ -35,8 +35,10 @@ All {ml} operations, such as creating and deleting {dfeeds}, jobs, and model
 snapshots.
 +
 --
-NOTE: Datafeeds run as a system user with elevated privileges, including
+NOTE: {dfeeds-cap} that were created prior to version 6.2 or created when {security}
-permission to read all indices.
+was disabled run as a system user with elevated privileges, including permission
+to read all indices. Newer {dfeeds} run with the security roles of the user who created
+or updated them.
 
 --
 
@@ -51,8 +53,10 @@ cache clearing.
 All watcher operations, such as putting watches, executing, activate or acknowledging.
 +
 --
-NOTE: Watches run as a system user with elevated privileges, including permission
+NOTE: Watches that were created prior to version 6.1 or created when {security}
-to read and write all indices.
+was disabled run as a system user with elevated privileges, including permission
+to read and write all indices. Newer watches run with the security roles of the user
+who created or updated them.
 
 --