Fix docs stating that savepoints are relocatable

This section from https://ci.apache.org/projects/flink/flink-docs-stable/ops/upgrading.html#preconditions states that savepoints are relocatable which they are not yet (see FLINK-5763). It should be fixed and/or removed; I'm unsure what change from 1.3 it should actually reflect.

Another important precondition is that for savepoints taken before Flink 1.3.x, all the savepoint data must be accessible from the new installation and reside under the same absolute path. Before Flink 1.3.x, the savepoint data is typically not self-contained in just the created savepoint file. Additional files can be referenced from inside the savepoint file (e.g. the output from state backend snapshots). Since Flink 1.3.x, this is no longer a limitation; savepoints can be relocated using typical filesystem operations..