DOC: set-log-level in Logging section preamble

As best I can tell, no mention is made of "set-log-level" in the Logging
[Section 8] of the doc.

Something akin to the following in the doc would have saved a good chunk of
time/angst in addressing a logging issue I encountered :
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 910dffc..1e01845 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -12945,6 +12945,8 @@
   - per-instance external troubles (servers up/down, max connections)
   - per-instance activity (client connections), either at the establishment or
     at the termination.
+  - per-request control of log-level, eg:
+        http-request set-log-level silent if sensitive_request
 
 The ability to distribute different levels of logs to different log servers
 allow several production teams to interact and to fix their problems as soon