[TASK] Added note about mixed setups (#4903)

* Added note about mixed setups

* Fixed whitespaces

Documentation/ApiOverview/SiteHandling/SiteSets.rst

@@ -169,6 +169,12 @@ be loaded multiple times, if a shared dependency is required by multiple sets.
     :typoscript:`@import` statements are still fine to be used for local
     includes, but should be avoided for cross-set/extensions dependencies.
 
+..  attention::
+    If the website uses a mixed setup consisting of a TypoScript template (`sys_template`)
+    and site sets, it is important to uncheck the "Clear" flag for constants and
+    setup in the TypoScript template. If the "Clear" flag is checked (default),
+    TypoScript settings from site sets are cleared and do therefore not apply.
+
 ..  _site-sets-page-tsconfig:
 
 Page TSconfig provider