XDCR connection pre-check documentation

XDCR connection pre-check is a new feature being introduced in 7.6 and also backported to 7.2.1.

The main ticket is MB-54382, with REST APIs documented more clearly in the UI ticket (MB-56860).

The UI work is underway for 7.6, but it is unclear at the time of the filing of this ticket whether or not it will make it to 7.2.1.

This documentation ticket is to track the followings:

1. This feature is introduced as part of 7.2.1, and is available in 7.6.
2. The REST APIs should at a minimum be documented as part of 7.2.1
3. Using the REST APIs should also be documented. The show-n-tell (https://docs.google.com/presentation/d/1mKBy4DDS9kr2_3yLNl_46FiALrjBU07yfKc4RsjkR90/edit#slide=id.gdf24278696_0_76) demonstrates example of how the REST APIs should be driven. Or, Sumukh Bhat can explain more.