docs: clarify distributed DiskSaver construction

[nightcityblade]

Fixes #2035

Description:

Clarify that Checkpoint and DiskSaver must be instantiated on every process in distributed configurations. Document that save_on_rank controls which process writes the checkpoint and that rank-guarded construction can block while Ignite discovers an externally initialized process group.

Tests:

• pre-commit run ruff-check --files ignite/handlers/checkpoint.py
• pre-commit run black --files ignite/handlers/checkpoint.py
• python -m pytest tests/ignite/handlers/test_checkpoint.py -q (66 passed, 4 skipped)

Check list:

• New tests are added (if a new feature is added) — N/A, documentation-only clarification
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)