Update jump step docs with cross-integration information

[stscijgbot-jp]

Issue JP-3892 was created on JIRA by Bryan Hilbert:

Recent improvements to the jump step include the ability to search for jumps across integrations within an exposure. i.e. [JP-2517|[https://github.com/spacetelescope/stcal/pull/174]? https://jira.stsci.edu/browse/JP-2517]
However, details of this behavior are not provided in the jump step documentation. 

From a recent conversation with David Law and Michael Regan:
if only_use_ints=True and nints > 100, do sigma clipping across integrations.
if only_use_ints=False and nints*ngroups>100, do sigma clipping using both groups and intsAnd then the options for finding 4 sigma outliers in each ramp, then the iterative flagging option.

It would be good to clarify in the docs exactly what happens for each situation, including more clear text in the only_use_ints description about what exactly it does.  Also as part of this documentation work, the two-point difference method is detailed in the [stcal RTD|#jump-algorithm]], but there is no mention of it in the [jwst RTD|[https://jwst-pipeline.readthedocs.io/en/latest/jwst/jump/description.html]], It would be good to include at least a link to the stcal RTD on the jwst page.