Document native single step options.

Now include both mechanisms suggested for native triggers in
single-privilege-mode systems, instead of just carefully counting
instruction.

Addresses #830.

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Improve single step description for different debug features

This commit updates the description of single step implementation in the
debugger_implementation.tex file. It now provides a more detailed and clear
explanation on how to single step through code running in the same privilege
mode as the debugger, depending on the availability of \FcsrTcontrolMpte,
\FcsrTcontrolMte, and automatically preventing \FcsrMcontrolSixAction=0 triggers
from matching when entering a trap handler.
(gpt-4)