Explanation of the SPRM acronym added to definition of the Sprm class

Change-Id: I536cd6ba2d531292d5148819dfd914b76f4b1111
Reviewed-on: https://gerrit.libreoffice.org/29032Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>

writerfilter/inc/dmapper/resourcemodel.hxx

@@ -347,7 +347,7 @@ public:
 };
 
 /**
-   An SPRM.
+   An SPRM: Section, Paragraph and Run Modifier
 
  */
 class Sprm