RapidPE Pastro updates

RapidPE pastro updates should be documented. It is not obvious if it should be included at https://emfollow.docs.ligo.org/userguide/analysis/parameter_estimation.html#sky-localization-and-parameter-estimation since it does not update skymap. Also if the updates are restricted to specific pipelines (e.g. only gstlal pastro), it should be documented that we do not always update pastro. cc: @caitlin.rose @vinaya.valsan