org-manual: Document :eval yes header argument value

* doc/org-manual.org (Limit code block evaluation): Document that
:eval can be set to "yes" to evaluate the code all the time without
prompt.

Reported-by: Charles Millar <millarc@verizon.net>
Link: https://orgmode.org/list/87v8uj6l6m.fsf@localhost

doc/org-manual.org

@@ -18084,6 +18084,10 @@ The =eval= header argument can limit evaluation of specific code
 blocks and =CALL= keyword.  It is useful for protection against
 evaluating untrusted code blocks by prompting for a confirmation.
 
+- =yes= ::
+
+  Org always evaluates the source code without asking permission.
+
 - =never= or =no= ::
 
   Org never evaluates the source code.