[master] 647d389c9 Mention dynamic parameter bounds changes in 6.4

[Dridi Boukelmoune]

commit 647d389c9a4803a58008d0e87066d60f741be8d8
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Fri Mar 6 11:00:06 2020 +0100

    Mention dynamic parameter bounds changes in 6.4

diff --git a/doc/sphinx/whats-new/changes-trunk.rst b/doc/sphinx/whats-new/changes-trunk.rst
index fbda9d6b9..d0221e616 100644
--- a/doc/sphinx/whats-new/changes-trunk.rst
+++ b/doc/sphinx/whats-new/changes-trunk.rst
@@ -23,6 +23,30 @@ varnishd
 Parameters
 ~~~~~~~~~~
 
+Some parameters have dependencies and those are better documented now. For
+example :ref:`ref_param_thread_pool_min` can't be increased above
+:ref:`ref_param_thread_pool_max`, which is now indicated as such in the
+manual.
+
+On a running Varnish instance the ``param.show`` command will display the
+actual minimum or maximum, but an attempt to ``param.set`` a parameter above
+or below its dynamic maximum or minimum will mention the failure's cause in
+the error message::
+
+    varnish> param.show thread_pool_reserve
+    200
+    thread_pool_reserve
+            Value is: 0 [threads] (default)
+            Maximum is: 95
+
+            [...]
+
+    varnish> param.set thread_pool_reserve 100
+    106
+    Must be no more than 95 (95% of thread_pool_min)
+
+    (attempting to set param 'thread_pool_reserve' to '100')
+
 **XXX changes in -p parameters**
 
 Other changes in varnishd