doc,cpp: Treat GPG_ERR_FULLY_CANCELED as canceled

* doc/gpgme.texi (GPG_ERR_FULLY_CANCELED): New.
* lang/cpp/src/context.cpp (Error::isCanceled): Also return true for
GPG_ERR_FULLY_CANCELED.

For internal purposes, GnuPG sometimes uses the error code
GPG_ERR_FULLY_CANCELED instead of GPG_ERR_CANCELED. From a user
perspective both values mean the same thing and should therefore be
treated identically.

• GnuPG-bug-id: T6510