rs6000: Add 'd' for doubleword variant of vector insert

rs6000: Add 'd' for doubleword variant of vector insert

When the "Vector Insert" section was added to the documentation,
the doubleword ('d') variant was omitted.  Add it.

2020-09-23  Paul A. Clarke  <pc@us.ibm.com>

gcc/
	* doc/extend.texi: Add 'd' for doubleword variant of
	vector insert instruction.