clean up documentation for linear_pool and linear_pool_quantile

[elray1]

Two items here:

• The docstrings for the task_id_cols parameter of these functions reference "the specified output_type_col and output_type_id_col", but those are not parameters of these functions. If we think we need them, should we add them? Otherwise, let's remove references to them from this documentation.
• There is a bunch of duplicated text in the documentation for these two functions (many of the function arguments are the same). It would be easier to maintain this if we used magical roxygen things to reuse the documentation. For example, that would make it so we only had to update the documentation for the task_id_cols parameter in one place.