exclude PLForecastingModule docs from custom rnn and blockrnn modules #4540

Job	Run time
tests (macos-latest, 3.9, all)	47m 47s
lint	43s
docs	9m 26s
check-examples (03-FFT-examples.ipynb)	3m 28s
tests (ubuntu-latest, 3.9, all)	31m 7s
check-examples (04-RNN-examples.ipynb)	7m 40s
check-examples (00-quickstart.ipynb)	6m 38s
check-examples (02-data-processing.ipynb)	6m 36s
check-examples (01-multi-time-series-and-covariates.ipynb)	7m 8s
	2h 0m 33s

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