Documentation updates (#2131)

* Update notes in documentation for memory sizing on 32-bit and 64-bit PHP versions

* Additional notes on the fact that PHPSpreadsheet does not change cell addresses when loading a spreadsheet using a Read Filter

docs/topics/memory_saving.md

@@ -1,6 +1,6 @@
 # Memory saving
 
-PhpSpreadsheet uses an average of about 1k per cell in your worksheets, so
+PhpSpreadsheet uses an average of about 1k per cell (1.6k on 64-bit PHP) in your worksheets, so
 large workbooks can quickly use up available memory. Cell caching
 provides a mechanism that allows PhpSpreadsheet to maintain the cell
 objects in a smaller size of memory, or off-memory (eg: on disk, in APCu,

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

@@ -139,6 +139,11 @@ $reader->setReadFilter( new MyReadFilter() );
 $spreadsheet = $reader->load("06largescale.xlsx");
 ```
 
+Read Filtering does not renumber cell rows and columns. If you filter to read only rows 100-200, cells that you read will still be numbered A100-A200, not A1-A101. Cells A1-A99 will not be loaded, but if you then try to call `getCell()` for a cell outside your loaded range, then PHPSpreadsheet will create a new cell with a null value.
+
+Methods such as `toArray()` assume that all cells in a spreadsheet has been loaded from A1, so will return null values for rows and columns that fall outside your filter range: it is recommended that you keep track of the range that your filter has requested, and use `rangeToArray()` instead.
+
+
 ### \PhpOffice\PhpSpreadsheet\Writer\Xlsx
 
 #### Writing a spreadsheet

docs/topics/reading-files.md

@@ -324,6 +324,10 @@ to read and process a large workbook in "chunks": an example of this
 usage might be when transferring data from an Excel worksheet to a
 database.
 
+Read Filtering does not renumber cell rows and columns. If you filter to read only rows 100-200, cells that you read will still be numbered A100-A200, not A1-A101. Cells A1-A99 will not be loaded, but if you then try to call `getCell()` for a cell outside your loaded range, then PHPSpreadsheet will create a new cell with a null value.
+
+Methods such as `toArray()` assume that all cells in a spreadsheet has been loaded from A1, so will return null values for rows and columns that fall outside your filter range: it is recommended that you keep track of the range that your filter has requested, and use `rangeToArray()` instead.
+
 ```php
 $inputFileType = 'Xls';
 $inputFileName = './sampleData/example2.xls';