updated var docs (#48957)

* updates var docs, fixes #48949

Co-Authored-By: bcoca <bcoca@users.noreply.github.com>

changelogs/fragments/doc_debug_var.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - document debug's var already having implicit moustaches

lib/ansible/modules/utilities/logic/debug.py

@@ -33,6 +33,8 @@ options:
   var:
     description:
       - A variable name to debug.  Mutually exclusive with the 'msg' option.
+      - Be aware that this option already runs in Jinja2 context and has an implicit ``{{ }}`` wrapping,
+        so you should not be using Jinja2 delimiters unless you are looking for double interpolation.
   verbosity:
     description:
       - A number that controls when the debug is run, if you set to 3 it will only run debug when -vvv or above