s390: Revise s390-specific assembler option descriptions

s390: Revise s390-specific assembler option descriptions

Reorder, reword, and complete the s390-specific option descriptions.
Align the formatting of s390-specific assembler options to that of the
general assembler options in "as --help".

While at it change a warning message to use the term "z/Architecture"
instead of the deprecated "esame" (ESA Modal Extensions or ESAME) one.

gas/
	* config/tc-s390.c: Revise s390-specific assembler option
	descriptions.

Reviewed-by: Andreas Krebbel <krebbel@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>