Docs: add note on system_filter forced expansion fail

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 1ab2bba0a8c9170920cebf7413444517a8cc854a..1bc2028d50637596802ff2da4a494154ec50e838 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -16885,6 +16885,9 @@ generates any deliveries to files or pipes, or any new mail messages, the
 appropriate &%system_filter_..._transport%& option(s) must be set, to define
 which transports are to be used. Details of this facility are given in chapter
 &<<CHAPsystemfilter>>&.
+.new
+A forced expansion failure results in no filter operation.
+.wen
 
 
 .option system_filter_directory_transport main string&!! unset