clarify copy-by-value distinction

[EmilHvitfeldt]

I found

cpp11/vignettes/motivations.Rmd

Line 68 in d8edde9

Copy-by-value works similarly to R, except that only copying when something is changed it _always_ copies.

a little hard to parse at first glance. I tried to make it a little more clear.

[jimhester]

Thanks, that sentence got mangled at some point during revision, at one point it actually did make sense