Doc: Update docs for testing for boolean fields (#18271) (#18292)

(cherry picked from commit 8f4014b)

Co-authored-by: Karen Metts <[email protected]>

Author: mergify[bot]

docs/reference/tips-best-practices.md

@@ -60,13 +60,15 @@ filter {
     # we use a "temporal" field with a predefined arbitrary known value that
     # lives only in filtering stage.
     add_field => { "[@metadata][test_field_check]" => "a null value" }
+  }
 
+filter {
+  mutate {
     # we copy the field of interest into that temporal field.
     # If the field doesn't exist, copy is not executed.
     copy => { "test_field" => "[@metadata][test_field_check]" }
   }
 
-
   # now we now if testField didn't exists, our field will have
   # the initial arbitrary value
   if [@metadata][test_field_check] == "a null value" {