[skip ci] Explain in comment why match default arm is emitted first

Zend/zend_compile.c

@@ -5297,6 +5297,8 @@ void zend_compile_match(znode *result, zend_ast *ast)
 	uint32_t cond_count = 0;
 	uint32_t *jmp_end_opnums = safe_emalloc(sizeof(uint32_t), arms->children, 0);
 
+	// The default arm is emitted first to avoid live range issues where the tmpvar for the
+	// arm result is freed even though it has not been initialized yet.
 	if (!has_default_arm) {
 		if (!uses_jumptable) {
 			zend_update_jump_target_to_next(opnum_default_jmp);