docs(website): fix incorrect example about the contains function (#22724)

* docs(website): fix incorrect example about the `contains` function

The `contains` function is now infallible and always returns a boolean value.
This commit fixes the example in the `transformation` guide.

* docs(website): add section on context-specific execution differences

* Update website/content/en/guides/level-up/transformation.md

Co-authored-by: Pavlos Rontidis <[email protected]>

* docs(website): remove trailing space

---------

Co-authored-by: Pavlos Rontidis <[email protected]>

Author: seunghunee

website/content/en/guides/level-up/transformation.md

@@ -138,8 +138,8 @@ transforms:
       # If the log message contains the phrase "Great Scott!", set the new field
       # "critical" to true, otherwise set it to false. If the "contains" function
       # errors, log the error (instead of aborting the script, as above).
-      if (is_critical, err = contains(.message, "Great Scott!"); err != null) {
-        log(err, level: "error")
+      if (is_critical = contains(.message, "Great Scott!"); is_critical) {
+        log("It contains 'Great Scott!'", level: "info")
       }
 
       .critical = is_critical
@@ -158,6 +158,15 @@ A few things to notice about this script:
   `.foo[0]`, `.foo.bar`, `.foo.bar[0]`, and so on to reference subfields, array
   indices, and more.
 
+{{< info >}}
+Note that VRL functions can behave differently depending on the execution context.
+For example, the `contains` function above is infallible when ran inside a Vector/Remap process if the compiler can detect that the type of `.message` is a string.
+The same code might behave differently when run in the [VRL Playground][urls.playground],
+VRL CLI, or when `schema.log_namespace` is set to `true`.
+
+[urls.playground]: https://playground.vrl.dev/?state=eyJwcm9ncmFtIjoiLmlzX2NyaXRpY2FsID0gY29udGFpbnMoLm1lc3NhZ2UsIFwiR3JlYXQgU2NvdHQhXCIpIiwiZXZlbnQiOnsibWVzc2FnZSI6IkdyZWF0IFNjb3R0ISJ9LCJpc19qc29ubCI6ZmFsc2UsImVycm9yIjpudWxsfQ%3D%3D
+{{< /info >}}
+
 If you stop and restart Vector, you should see log lines like this (again
 reformatted for readability):