update doc on custom mode
While it is stated in the header file that The mode MUST NOT BE DESTROYED until the encoders and decoders that use it are destroyed as well. Right below the the mode definiton.
In the generated html doc they are far apart, this makes it easy to misinterpret that the mode is only used for initialization.
The encode api returned BadArg when the mode had been destroyed.
Edited by Marcus Asteborg