Document that update-policy external is synchronous

(cherry picked from commit 7d352741a0c7e54d158d8421532158df42868b9b)

diff --git a/doc/arm/reference.rst b/doc/arm/reference.rst
index 825e8153c563896b8c1140771f9a91825d682c00..493ce637b6942380f8a0ccc2942a185c67c16737 100644 (file)
--- a/doc/arm/reference.rst
+++ b/doc/arm/reference.rst
@@ -6034,6 +6034,8 @@ The ruletype field has 16 values: ``name``, ``subdomain``, ``zonesub``, ``wildca
 
     The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.
 
+       .. warning:: The external daemon must not delay communication. This policy is evaluated synchronously; any wait period negatively affects :iscman:`named` performance.
+
 .. _multiple_views:
 
 Multiple Views