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/storage/File.dox

@@ -280,7 +280,7 @@
 
 
 /*!
-	ssize_t BFile::Read(void* buffer, size_t size)
+	\fn ssize_t BFile::Read(void* buffer, size_t size)
 	\brief Reads a number of bytes from the file into a buffer.
 
 	\param buffer The buffer the data from the file shall be written to.

docs/user/storage/MimeType.dox

@@ -906,7 +906,6 @@
 	\retval B_OK The ref was retrieved successfully.
 	\retval B_NO_INIT BMimeType was uninitialized.
 	\retval B_BAD_VALUE \a ref was \c NULL.
-	\retval B_NO_INIT BMimeType was uninitialized.
 
 	\since BeOS R3
 */