scsi: docs: Remove invalid link and update text for zfcp kernel config

scsi: docs: Remove invalid link and update text for zfcp kernel config

IBM decided to retire a lot of the content that was previously hosted on
"developerworks", and so some of the links we've used for documentation are
now dead or redirect to some general landing page with no correlation to
what the links were meant to provide.

Change the provided link in the Kconfig file for zfcp to rather refer to
our device drivers book that we regularly update and publish for free, and
whose name hasn't been changed since it was first published.

Our hardware is also not called "IBM eServer zSeries" anymore - in fact, it
hasn't been called like that since 2006. Use a broader term that covers
different server names over time.

Lastly, add a short paragraph about how our HBAs are typically named, to
have some more tangible references.

Link: https://lore.kernel.org/r/96069b9f4c4f056a515b37e89b2bdfccc282e3d3.1593780621.git.bblock@linux.ibm.com
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>