[docs] add stipulation to SetURLVars that it should only be used for testing purposes

Author: Zakariah Chitty

test_helpers.go

@@ -6,11 +6,13 @@ package mux
 
 import "net/http"
 
-// SetURLVars sets URL variables. This can be used to simplify the testing of
-// request handlers.
-// Alternatively, URL variables can be set by making a route that captures the
-// required variables, starting a server and sending the request to that
-// server.
+// SetURLVars sets the URL variables for the given request, to be accessed via
+// mux.Vars for testing route behaviour.
+//
+// This API should only be used for testing purposes; it provides a way to
+// inject variables into the request context. Alternatively, URL variables
+// can be set by making a route that captures the required variables,
+// starting a server and sending the request to that server.
 func SetURLVars(r *http.Request, val map[string]string) *http.Request {
 	return setVars(r, val)
 }