Docs: remove roxygen warnings, register non-exported S3 methods

[markheckmann]

Change: registered non-exported S3 methods.

Roxygen complaints because of non-exported S3 methods.

ℹ Updating officer documentation
ℹ Loading officer
✖ core_properties.R:54: S3 method `[<-.core_properties` needs @export or
  @exportS3method tag.
✖ custom_properties.R:53: S3 method `[.custom_properties` needs @export or
  @exportS3method tag.
... 

These have to be exported as documented here:

You must register, i.e. @export, every S3 method regardless of whether or not the generic is exported. roxygen2 will warn you if you have forgotten.

This does not generate new .Rd entries. See also this discussion for details.