Clarified formatting multiple values doc

docs/sources/reference/templating.md

@@ -174,6 +174,8 @@ Interpolating a variable with multiple values selected is tricky as it is not st
 is valid in the given context where the variable is used. Grafana tries to solve this by allowing each data source plugin to
 inform the templating interpolation engine what format to use for multiple values.
 
+Note that the *Custom all value* option on the variable will have to be left blank for Grafana to format all values into a single string.
+
 **Graphite**, for example, uses glob expressions. A variable with multiple values would, in this case, be interpolated as `{host1,host2,host3}` if
 the current variable value was *host1*, *host2* and *host3*.