add missing values handling templates for model documentation (#1290)

Author: focardozom

man-roxygen/na_casewise_delete.R

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+#' @section Missing Values:
+#' This model silently removes rows with missing values (case-wise deletion) before fitting.

man-roxygen/na_errors.R

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+#' @section Missing Values:
+#' This model fails with an error if any rows contain missing values. Missing values should be handled before fitting.

man-roxygen/na_handled.R

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+#' @section Missing Values:
+#' This model natively handles missing values during training.