sdf 1.8: Improve docs of collision_bitmask.

* added to sdf 1.7 in gazebosim#521

Signed-off-by: Martin Pecka <peckama2@fel.cvut.cz>
Signed-off-by: Steve Peters <scpeters@openrobotics.org>

sdf/1.8/surface.sdf

@@ -141,11 +141,11 @@
     </element>
 
     <element name="collide_bitmask" type="unsigned int" default="65535" required="0">
-      <description>Bitmask for collision filtering. This will override collide_without_contact</description>
+      <description>Bitmask for collision filtering. This will override collide_without_contact. Parsed as 16-bit unsigned integer.</description>
     </element>
 
     <element name="category_bitmask" type="unsigned int" default="65535" required="0">
-      <description><![CDATA[Bitmask for category of collision filtering. Collision happens if ((category1 & collision2) | (category2 & collision1)) is not zero. If not specified, the category_bitmask should be interpreted as being the same as collide_bitmask.]]></description>
+      <description><![CDATA[Bitmask for category of collision filtering. Collision happens if ((category1 & collision2) | (category2 & collision1)) is not zero. If not specified, the category_bitmask should be interpreted as being the same as collide_bitmask. Parsed as 16-bit unsigned integer.]]></description>
     </element>
 
     <element name="poissons_ratio" type="double" default="0.3" required="0">