Document immutable expression attributes

Signed-off-by: Christoph Knittel <ck@cca.io>

Author: cknitt

CHANGELOG.md

@@ -32,6 +32,7 @@
 
 #### :house: Internal
 
+- Make expression attributes immutable in the current parsetree, now that editor refactors construct new expression nodes. https://github.com/rescript-lang/rescript/pull/8685
 - Remove the unused `pat_record_label` alias from the current parsetree. https://github.com/rescript-lang/rescript/pull/8684
 - Remove the unused `Ppat_open` and `Tpat_open` AST nodes, with current-AST and CMT format bumps. https://github.com/rescript-lang/rescript/pull/8682
 - Represent ternary expressions with a dedicated parsetree node. https://github.com/rescript-lang/rescript/pull/8674