docs(customization): cut the callback's prose to what a reader needs

callbacks.py had grown to 484 lines of comment and docstring against 177 of
code. Most of the excess was narrative -- what an earlier version did, what
was tried and removed, the same point restated from a second angle -- which
belongs in the design note, not above the function.

Cut to 322. What stays is what a reader would otherwise get wrong: the
merge semantics that make a sent key destructive, why the phase prefix is
load-bearing, why `fnmatchcase` and not `fnmatch`, why report_training_start
states neither step nor metrics, why a null checkpoint_path is omitted
rather than sent. Measured numbers stay; the essays justifying them now
point at AALGO-497.

Verified prose-only: the module's AST with every docstring stripped is
byte-identical before and after.

Signed-off-by: Albert Cui <albcui@nvidia.com>

Author: albcui