Add documentation for the SBTarget::ReadInstructions and
SBTarget::GetInstructions APIs so it's a little clearer to 
understand which should be used.

<rdar://problem/18787018>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222225 91177308-0d34-0410-b5e6-96231b3b80d8