bootloader docs inconsistent

There are two locations in the docs that discuss the nature of the images to be run by the bootloader.

./boot/bootutil/design.txt: * Build to run from a fixed location (i.e., not position-independent).
./docs/os/modules/bootloader/bootloader.rst:- Build to run from a fixed location (i.e., position-independent).

I believe that the first one (bootutil/design.txt) is the correct statement (well, at least it's internally self-consistent).

Compare boot/bootutil/design.txt with docs/os/modules/bootloader/bootloader.rst, focusing on the conflicting statements about whether images run from a fixed location. Update the inconsistent documentation so both locations state the same correct behavior, then verify the two passages match.

Written by the indexing model from the issue text.