doc: Update Power builtin documentation in user's manual

The standard for many Power vector interfaces is now the recently
published Power Vector Intrinsic Programming Reference.  Reference
that document for the relevant interfaces, and remove redundant
information from the GCC user's manual.

2021-04-16  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* doc/extend.texi (PowerPC AltiVec/VSX Built-in Functions): Revise
	this section and its subsections.