Add clarification to avoid misunderstanding that cause #3360 (#3378)

* Update macros.md

Typos, typos

doc/mkdocs/docs/features/macros.md

@@ -51,6 +51,8 @@ that enabling this macro increases the size of every JSON value by one pointer a
 The diagnostics messages can also be controlled with the CMake option `JSON_Diagnostics` (`OFF` by default) which sets
 `JSON_DIAGNOSTICS` accordingly.
 
+WARNING: As this macro changes the definition of the json object, it MUST be defined in the same way globally, even across different compilation units; do NOT link together code compiled with and without JSON_DIAGNOSTICS defined as this is a violation of the One Definition Rule and will cause undefined behaviour.
+
 !!! info "Default behavior"
 
     ```cpp