Documentation for @noRd functions

There are a few functions in `climr` that have little or no documentation and for which no documentation file (the .Rd file) is built (`@noRd`).

I think that it would be useful to try to 9almost) fully document these functions (description, parameters and imports), even if it's not going to be visible to package users, as this could facilitate future development (especially by new developers).

What do you think @kdaust, @cmahony ?

Search the package source for functions marked with @noRd and identify which have little or no documentation. Add descriptions, parameters, and imports for those functions, then verify that the documentation is complete even though no .Rd files are built for them.

Written by the indexing model from the issue text.