Update spacing.md - Add commit about pt to twip conversion

Author: rasamassen

docs/usage/styles/spacing.md

@@ -11,8 +11,8 @@ $section->addText('Hello, World!', null, 'pStyle');
 See [`Sample_08_ParagraphPagination`](/samples/Sample_08_ParagraphPagination.php) for more code samples.
 
 ## Options
-- `after`. Space after paragraph in *twip*.
-- `before`. Space before paragraph in *twip*.
+- `after`. Space after paragraph in *twip*. 1pt = 20twip.
+- `before`. Space before paragraph in *twip*. 1pt = 20twip.
 - `line`. Space between lines within a paragraph in *twip*. If lineRule is auto, 240 (height of 1 line) will be added, so if you want a double line height, set this to 240.
 - `lineRule`. Line Spacing Rule. *auto*, *exact*, *atLeast*
    * See [`SimpleType > LineSpacingRule`](../simpletypes/linespacingrule.md) class constants for possible values.