[lldb] Update qRegisterInfo docs to recommend target.xml (#69853)

Before target.xml, lldb had its own method for querying the remote stub
of the registers it supports, qRegisterInfo. The gdb standard method of
using a target.xml file to describe the available registers has become
commonplace, and the lldb method for doing this is no longer needed.

Stubs should describe their registers to lldb, but it should be with the
target.xml file.