HaikuBook: fix warnings generated by Doxygen 1.18.16.

This version generates some warnings and identified some problems in our
documentation. It is also a bit more vigilant about undocumented elements, so
these are now decorated with a brief description. In the BNotification docs I
used a stray \p to create some whitespace between code and an image. This is
now fixed in the CSS instead.

docs/user/interface/Shape.dox

@@ -447,11 +447,6 @@
 	\retval B_OK The operation was added successfully.
 	\retval B_NO_MEMORY Ran out of memory while trying to add the operation.
 
-	\return Returns a status code, \c B_OK on success or an error code
-	        otherwise.
-	\retval B_OK The operation was added successfully.
-	\retval B_NO_MEMORY Ran out of memory while trying to add the operation.
-
 	\since BeOS R4
 */