Extend the Options section of the PDF Writer documentation (#2642)

* Extend the Options section of the PDF Writer documentation

When I used PHPWord to write a PDF document, recently, I followed the
docs only to find that I also had to specify a PDF renderer — otherwise
the operation failed.

After a bit of research to figure out what to do to specify one, I
thought it only right to document what I learned in the official
documentation.

* Update the docs changelog detailing the PDF Writer additions

This change updates the changelog to include the enhancement made to the
PDF Writer docs that show how to specify a PDF renderer when working
with the PDF writer.

* Fixed docs

---------

Co-authored-by: Progi1984 <progi1984@gmail.com>

Author: settermjd

docs/changes/2.x/2.0.0.md

@@ -5,6 +5,7 @@
 ## Enhancements
 
 - IOFactory : Added extractVariables method to extract variables from a document [@sibalonat](https://github.com/sibalonat) in [#2515](https://github.com/PHPOffice/PHPWord/pull/2515)
+- PDF Writer : Documented how to specify a PDF renderer, when working with the PDF writer, as well as the three available choices by [@settermjd](https://github.com/settermjd) in [#2642](https://github.com/PHPOffice/PHPWord/pull/2642)
 
 ### Bug fixes
 

docs/usage/writers.md

@@ -87,6 +87,29 @@ $writer = IOFactory::createWriter($oPhpWord, 'PDF');
 $writer->save(__DIR__ . '/sample.pdf');
 ```
 
+#### Specify the PDF Renderer
+
+Before PHPWord can write a PDF, you **must** specify the renderer to use and the path to it.
+Currently, three renderers are supported: 
+
+- [DomPDF](https://github.com/dompdf/dompdf)
+- [MPDF](https://mpdf.github.io/)
+- [TCPDF](https://tcpdf.org/)
+
+To specify the renderer you use two static `Settings` functions:
+
+- `setPdfRendererName`: This sets the name of the renderer library to use.
+  Provide one of [`Settings`' three `PDF_` constants](https://github.com/PHPOffice/PHPWord/blob/master/src/PhpWord/Settings.php#L39-L41) to the function call.
+- `setPdfRendererPath`: This sets the path to the renderer library. 
+  This directory is the renderer's package directory within Composer's _vendor_ directory.
+
+In the code below, you can see an example of setting MPDF as the desired PDF renderer.
+
+```php
+Settings::setPdfRendererName(Settings::PDF_RENDERER_MPDF);
+Settings::setPdfRendererPath(__DIR__ . '/../vendor/mpdf/mpdf');
+```
+
 ## RTF
 The name of the writer is `RTF`.