Docs: Highlight need for explicit import of DataFlow lib

at least in some langauges

Author: RasmusWL

docs/codeql/writing-codeql-queries/creating-path-queries.rst

@@ -55,6 +55,8 @@ You should use the following template:
      */
 
     import <language>
+    // For some languages (Java/C++/Python) you need to explicitly import the data flow library, such as
+    // import semmle.code.java.dataflow.DataFlow
     import DataFlow::PathGraph
     ...