Document That Precision As Displayed Is Not Supported

Fix #1247. Aside from being difficult to implement, see, for example, https://answers.microsoft.com/en-us/msoffice/forum/all/what-are-the-dangers-of-using-precision-as/1a2ba5eb-9470-452f-816e-d7961c0d0ad5

Author: oleibman

docs/topics/calculation-engine.md

@@ -136,6 +136,10 @@ This is normal behaviour of the compatibility pack, Xlsx displays this
 correctly. Use `\PhpOffice\PhpSpreadsheet\Writer\Xls` if you really need
 calculated values, or force recalculation in Excel2003.
 
+#### PAD (Precision As Displayed) Not Supported
+
+There are no plans to support Precision As Displayed.
+
 ## Handling Date and Time Values
 
 ### Excel functions that return a Date and Time value