Add @method roxygen statements to "early" `knit_print` methods

[Puzzled-Face]

Whilst I developed the knit_print methods, I realised that the way I had initially documented the methods was sub-optimal. The recommended ay to implement an S3 method for an S4 class is

#' @method <method name> <class name>

Method documentation in helpers_knitr_Increments.R, helpers_knitr_CohortSize.R and helpers_knitr_GeneralData.R should be updated.

This is a sub task of #774.

[Famous last words] This should be straightforward.