8bbe0bc838
d2a802d3c6 got overzealous by removing the "tweak" property which was
actually necessary to differentiate box select on click-drag vs. box
select on B. Fix by restoring the tweak property for everything but
preview keymap items (where it doesn't actually do anything).
Main reason this happened is because the original description was a bit
too cryptic, so update it to be clearer.
Pull Request: https://projects.blender.org/blender/blender/pulls/130015