Update Changelog for switch from built-in PHP functions to Excel class methods (to handle typechecking for stricter PHP8 typing)

CHANGELOG.md

@@ -13,7 +13,14 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Changed
 
-- Nothing.
+- Formula functions that previously called PHP functions directly are now processed through the Excel Functions classes; resolving issues with PHP8 stricter typing. [#1789](https://github.com/PHPOffice/PhpSpreadsheet/issues/1789)
+
+  The following MathTrig functions are affected:
+  `ABS()`, `ACOS()`, `ACOSH()`, `ASIN()`, `ASINH()`, `ATAN()`, `ATANH()`,
+  `COS()`, `COSH()`, `DEGREES()` (rad2deg), `EXP()`, `LN()` (log), `LOG10()`,
+  `RADIANS()` (deg2rad), `SIN()`, `SINH()`, `SQRT()`, `TAN()`, `TANH()`.
+  
+  One TextData function is also affected: `REPT()` (str_repeat).
 
 ### Deprecated