Warn users that `DumbCheckpoint` should be used sparingly

[ReubenHill]

Following the merge of #2084, which silently broke back compatibility with certain DumbCheckpoint files, it has become apparent that DumbCheckpoint is intended to be used sparingly and only within individual firedrake "sessions" (i.e. certainly not between firedrake-updates). It certainly should never be used as an archival format.

This should be clearly documented in the manual - at the moment it seems (to me at least) like the obvious in-built HDF5 I/O format. (Recent improvements to our plotting capability make its use particularly attractive - one can run an experiment, save the results as a DumbCheckpoint then load them in again for plotting and analysis in python).

[ReubenHill]

Perhaps my use case (saving for plotting) falls under the intended use but the point to make clear in the documentation is that it should absolutely not be used for archival?