Improving lli documentation

Too many people hope lli would act as an emulator when it's actually
just a tool to help prototype IR code and test the JIT compiler. This
commit makes that fact explicit in the documentation

It also migrates the old style bold/italic doc tags to the preferred
meta tags (.. option::, :program:, etc).

No errors when generating the documents, visual inspection in the HTML
result doesn't show any major difference, apart from the slight style
change.

llvm-svn: 243401