[StreamExecutor] Fix KernelSpec Doxygen

Summary:
There was a typo where \endcode was spelled as \encode and it was
keeping the whole file document from rendering. I also added in some \c
annotations for inline code stuff to make it look nicer.

Reviewers: jprice

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D23941

llvm-svn: 279855