There are two kind of materialization callbacks: one for target
materializations and one for source materializations. The callback type
for target materializations is `TargetMaterializationCallbackFn`. This
commit renames the one for source materializations from
`MaterializationCallbackFn` to `SourceMaterializationCallbackFn`, for
consistency.
There used to be a single callback type for both kind of
materializations, but the materialization function signatures have
changed over time.
Also clean up a few places in the documentation that still referred to
argument materializations.
fc8484f0e3