Add extract_postprocessor() to docs for extract_*() functions

[hfrick]

It should also be listed in the @description

hardhat/R/extract.R

Lines 3 to 17 in ae8fba7

	#' @description
	#' These generics are used to extract elements from various model
	#' objects. Methods are defined in other packages, such as tune,
	#' workflows, and workflowsets, but the returned object is always the same.
	#'
	#' - `extract_fit_engine()` returns the engine specific fit embedded within
	#' a parsnip model fit. For example, when using `parsnip::linear_reg()`
	#' with the `"lm"` engine, this returns the underlying `lm` object.
	#'
	#' - `extract_fit_parsnip()` returns a parsnip model fit.
	#'
	#' - `extract_mold()` returns the preprocessed "mold" object returned
	#' from [mold()]. It contains information about the preprocessing,
	#' including either the prepped recipe, the formula terms object, or
	#' variable selectors.