Quite a lot of improvements in the native_posix board
documentation:
* Added TOC
* Reordered
* Given much more emphasis to its limitation
* Added subsection explaining how to overcome some of these
limitations
* Added rationale section
* Added section comparing this port with QEMU and ISSs
* Corrected build example (this port cannot be compiled on
Windows, regression from commit
e62ee6ab3c )
* Added note about it not working on W10 WSL (#5762)
* Several other minor fixes
Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>