Document Charset Restriction for Html/Xml Reader (#3068)

Fix #1681, although probably not to the originator's satisfaction. Html and Xml readers will handle documents only with a charset of UTF-8. This PR documents that restriction. No change to source code; see the original issue for explanation.

Author: oleibman

docs/topics/reading-and-writing-to-file.md

@@ -282,6 +282,7 @@ versions of Microsoft Excel.
 **Excel 2003 XML limitations** Please note that Excel 2003 XML format
 has some limits regarding to styling cells and handling large
 spreadsheets via PHP.
+Also, only files using charset UTF-8 are supported.
 
 ### \PhpOffice\PhpSpreadsheet\Reader\Xml
 
@@ -701,6 +702,7 @@ extension.
 
 **HTML limitations** Please note that HTML file format has some limits
 regarding to styling cells, number formatting, ...
+Also, only files using charset UTF-8 are supported.
 
 ### \PhpOffice\PhpSpreadsheet\Reader\Html