Document ModelCheckpoint.file_exists DDP memory fix

Author: littlebullGit

src/lightning/pytorch/CHANGELOG.md

@@ -82,6 +82,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed FSDP mixed precision semantics and added user warning ([#21361](https://github.com/Lightning-AI/pytorch-lightning/pull/21361))
 
 
+- Fixed `ModelCheckpoint.file_exists` using broadcast in DDP, reducing memory usage when checking for existing checkpoints ([#19674](https://github.com/Lightning-AI/pytorch-lightning/issues/19674))
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 ## [2.5.6] - 2025-11-05