docs: tools should only interpret a line starting with { as JSON

src/doc/src/reference/external-tools.md

@@ -46,6 +46,10 @@ The `package_id` field is a unique identifier for referring to the package, and
 as the `--package` argument to many commands. The syntax grammar can be found in
 chapter [Package ID Specifications].
 
+> **Note:** Not all output is guaranteed to be in JSON format with the `--message-format=json` option.
+> For example, output from procedural macros and logs from external tools may appear in plain text.
+> Tools should only interpret a line as JSON if it starts with `{`.
+
 The `--message-format` option can also take additional formatting values which
 alter the way the JSON messages are computed and rendered. See the description
 of the `--message-format` option in the [build command documentation] for more