doc: document %pop with argument

Document that %pop can now take an argument, and what it does.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/doc/changes.src b/doc/changes.src
index 86ba95e..5c756a1 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -12,6 +12,8 @@
 
 \b Support for indirect macro expansion (\c{%[...]}).  See \k{indmacro}.
 
+\b \c{%pop} can now take an argument, see \k{pushpop}.
+
 
 \S{cl-2.05} Version 2.05