Update documentation to indicate supported upgrades

Please see MB-59786.
7.2.4 upgraded Erlang/OTP version to 25.

Documentation/release notes should reflect the change in behavior from 7.2.3 to 7.2.4:

• A node running 7.2.4 is incompatible (i.e. cannot communicate) with a node running 6.5.x, 6.6.x and 7.0.x). A node running 7.2.4 is compatible only with nodes running 7.1 or later.

(A node running 7.2.3, on the other hand, is compatible with nodes running 6.5.x or later).

• Direct upgrades are not supported from pre-7.1 to 7.2.4. A node running 6.5.x, 6.6.x, 7.0.x must be upgraded to 7.1 first, before an upgrade to 7.2.4.

(Direct upgrades were supported from 6.5.x to 7.2.3).