doc: native_posix: Add section about 32 and 64 bits

native_posix supports for some time now generating a 64bit
binary with the native_posix_64 target, but we never got
to document it.
Add a section describing it, and correct the WSL1
note/warning (also with a link with instructions on how to hack
WSL1 to make it run 32 bit binaries)

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>