[PowerPC] Document requirements for VSX feature

[PowerPC] Document requirements for VSX feature

As suggested in
https://sourceware.org/ml/gdb-patches/2018-10/msg00510.html, this
patch changes the documentation for the VSX tdesc feature to make it
clear that the altivec and FPU features are requirements.

gdb/doc/ChangeLog:
2018-11-09  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* gdb.texinfo (PowerPC Features): Document the altivec and fpu
	requirements for the org.gnu.gdb.power.vsx feature.