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/Button.dox

@@ -530,7 +530,7 @@
 
 
 /*!
-	\fn status_t BControl::SetIcon(const BBitmap* icon, uint32 flags)
+	\fn status_t BButton::SetIcon(const BBitmap* icon, uint32 flags)
 	\brief This convenience method is used to set the bitmaps
 	       for the standard states from a single bitmap.
 

docs/user/interface/Region.dox

@@ -187,7 +187,7 @@
 
 
 /*!
-	fn clipping_rect BRegion::RectAtInt(int32 index)
+	\fn clipping_rect BRegion::RectAtInt(int32 index)
 	\brief Returns the clipping_rect contained in the region at the given
 	       \a index.
 

docs/user/interface/Screen.dox

@@ -452,7 +452,6 @@
 	       available display modes.
 
 	\retval B_OK if the operation was successful.
-	\retval B_ERROR if \a modeList or \a count is invalid.
 	\retval B_ERROR for all other errors.
 
 	\since BeOS R5

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
 */