Reuse description of problem being solved by tar_terra_* family of targets functions

[njtierney]

As noted in the review in ropensci/software-review#675

• The details section of the tar_terra_rast (and tar_terra_vect) documentation gives a nice brief explanation of the problem the function is solving and how it addresses it. Other tar_terra_* functions (such as tar_terra_sds) do not contain a similar explanation, and would benefit from having it, in accordance with the multiple points of enrty principle.

I am now rewriting some of the documentation so it can be reused (https://cran.r-project.org/web/packages/roxygen2/vignettes/reuse.html) in multiple places.