userguide/upgrade: add note about alerts' increase

With triggering stream reassembly early, since for certain types of
rules there may be more alerts triggered - even in IPS mode, make this
clear in the upgrading section.

Bug #7026

doc/userguide/upgrade.rst

@@ -57,6 +57,9 @@ Major changes
   Instead, both the SDP parser and logger depend on being invoked by another parser (or logger).
 - ARP decoder and logger have been introduced.
   Since ARP can be quite verbose and produce many events, the logger is disabled by default.
+- It is possible to see an increase of alerts, for the same rule-sets, if you
+  use many stream/payload rules, due to Suricata triggering TCP stream
+  reassembly earlier.
 
 Upgrading 6.0 to 7.0
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