Docs: Update description for Sphinx SVG plugin

Correct the description which is copied from plantuml,
and update the missing information.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: Iaeecc0f481db1ef94e3a35920865e3dca467feda
diff --git a/docs/conf.py.in b/docs/conf.py.in
index 99b2fcb..b4430c9 100644
--- a/docs/conf.py.in
+++ b/docs/conf.py.in
@@ -50,7 +50,7 @@
     'm2r', #Support markdown files. Needed for external code.
     'sphinx.ext.autosectionlabel', #Make sphinx generate a label for each section
     'sphinxcontrib.plantuml', #Add support for PlantUML drawings
-    'sphinxcontrib.rsvgconverter' #Add support for PlantUML drawings
+    'sphinxcontrib.rsvgconverter' #Add support for SVG to PDF
 ]
 
 #Location of PlantUML