s390/docs: Break long lines in Debugging390.txt

There are a lot of lines that are longer than 80 columns in this file,
rendering it hard to read in a terminal window. This patch fixes most
of these long lines, and while we're at it, also makes some sentences
more readable, e.g. by replacing "&" with "and", adding proper
punctuation, removing superfluous clauses, etc.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>