Don't remove buildroot docdir on %doc usage (ticket #836)

Don't remove buildroot docdir on %doc usage (ticket #836)
- Some software installs its own documentation and if you try to
  combine it with %doc, rpmbuild will abort with mysterious
  "cpio bad magic" or such errors because what was assumed to be
  there was not, as %doc would 'rm -rf' the docdir upon first
  invocation. I don't see any good reason to disallow sharing the
  same directory for %doc and documentation installed by software
  "make install" - the other alternative would be forcing %doc to
  go to some other location, only making docs harder to find.
- Also at least Mageia (and prior to that Mandriva) has been doing
  this for quite some time now.