[TASK] Add further hint on backend/frontend tree.level, fix indentation

Documentation/UsingSetting/Conditions.rst

@@ -165,12 +165,17 @@ Example: Condition applies on a page on root level
         // Your settings go here
     [END]
 
-    ..  hint::
+..  hint::
 
-        In older versions before TYPO3 v10, this setting was available as
-        `treeLevel` variable.
-        That variable started the root level at value `0`, and now it starts at `1`.
-        Keep this in mind when migrating old conditions.
+    In older versions before TYPO3 v10, this setting was available as
+    `treeLevel` variable.
+    That variable started the root level at value `0`, and now it starts at `1`.
+    Keep this in mind when migrating old conditions.
+
+    Unlike the frontend TypoScript condition `tree.level`, the backend tree level
+    is not affected by `pages.is_siteroot`. This means, for nested sites it cannot
+    be used to match "site roots" or in general any n-th levels of page tree depths.
+    It's the *absolute depth* of the entire page tree, starting with `1`.
 
 ..  index:: Conditions; tree.pagelayout
 ..  _condition-tree-pagelayout: