Clean up the doxygen formatting of the comments on the strip* methods on

Clean up the doxygen formatting of the comments on the strip* methods on
Value. These methods probably don't belong here, and I'm discussing
moving the lot of them to a better home, but for now I'm about to extend
their functionality and wanted to tidy them up first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188997 91177308-0d34-0410-b5e6-96231b3b80d8