Miscellaneous documentation corrections

[advieser]

PipeOpTaskPreprocSimple has the same construction as PipeOpTaskPreproc. However, tags and feature_types are only documented for PipeOpTaskPreproc.

[advieser]

PipeOpDecode is missing Fields documentation.

#' @section Fields:
#' Only fields inherited from [`PipOpTaskPreprocSimple`]/[`PipeOpTaskPreproc`]/[`PipeOp`].

Same for PipeOpEncode or check in which cases we do not list the Fields section.

[advieser]

Add backticks for some Fields of PipeOp