s390: Document syntax to omit base register operand

Document the s390-specific assembler syntax introduced by commit
aacf780b ("s390: Allow to explicitly omit base register operand in
assembly") to omit the base register operand B in D(X,B) and D(L,B) by
coding D(X,) and D(L,).

While at it document the alternative syntax to omit the index register
operand X in D(X,B) by coding D(,B) instead of D(B).

gas/
	* doc/c-s390.texi (s390 Operands): Document syntax to omit base
	register operand.

Fixes: aacf780b ("s390: Allow to explicitly omit base register operand in assembly")
Signed-off-by: Jens Remus <jremus@linux.ibm.com>