Editorial: Add notes about lifetime of surrounding agent

These notes were missing from GetIANATimeZonePreviousTransition and
GetIANATimeZoneNextTransition. All operations dealing with the
implementation's internal time zone database need to have this note.

spec/timezone.html

@@ -514,6 +514,9 @@ <h1>
       <p>
         The returned value represents the number of nanoseconds since the Unix epoch in UTC that corresponds to the first time zone transition after _epochNanoseconds_ in the IANA time zone identified by _timeZoneIdentifier_, or *null* if no such transition exists.
       </p>
+      <p>
+        Given the same values of _epochNanoseconds_ and _timeZoneIdentifier_, the result must be the same for the lifetime of the surrounding agent.
+      </p>
     </emu-clause>
 
     <emu-clause id="sec-temporal-getianatimezoneprevioustransition" type="implementation-defined abstract operation">
@@ -528,6 +531,9 @@ <h1>
       <p>
         The returned value represents the number of nanoseconds since the Unix epoch in UTC that corresponds to the last time zone transition before _epochNanoseconds_ in the IANA time zone identified by _timeZoneIdentifier_, or *null* if no such transition exists.
       </p>
+      <p>
+        Given the same values of _epochNanoseconds_ and _timeZoneIdentifier_, the result must be the same for the lifetime of the surrounding agent.
+      </p>
     </emu-clause>
 
     <emu-clause id="sec-temporal-parsetimezoneoffsetstring" aoid="ParseTimeZoneOffsetString">