[docs][llvm-objcopy][llvm-strip] Improve --strip-unneeded description

Behaviour was recently added to this switch to strip debug sections too.
See r369761.

This change also makes the description for the --strip-unneeded switch
consistent between the two docs.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D67546

llvm-svn: 371855