fix: clarify vacuum command documentation for DeltaTable (delta-io#4196)

# Description
Improve documentation for vacuum command and specifically for
**full=False** case.

# Related Issue(s)
Related to this discussion:
delta-io#3644

# Documentation
Updated

Signed-off-by: khalidmammadov <khalidmammadov9@gmail.com>

Author: khalidmammadov

python/deltalake/table.py

@@ -554,15 +554,18 @@ def vacuum(
         keep_versions: list[int] | None = None,
     ) -> list[str]:
         """
-        Run the Vacuum command on the Delta Table: list and delete files no longer referenced by the Delta table and are older than the retention threshold.
+        Run the Vacuum command on the Delta Table: list and delete files no longer referenced by the Delta table.
+        Here "not referenced" means all removed files (from vacuum/delete/update/merge) older than the retention threshold,
+        plus any files not mentioned in the logs (unless they start with underscore).
 
         Args:
             retention_hours: the retention threshold in hours, if none then the value from `delta.deletedFileRetentionDuration` is used or default of 1 week otherwise.
             dry_run: when activated, list only the files, delete otherwise
             enforce_retention_duration: when disabled, accepts retention hours smaller than the value from `delta.deletedFileRetentionDuration`.
             post_commithook_properties: properties for the post commit hook. If None, default values are used.
             commit_properties: properties of the transaction commit. If None, default values are used.
-            full: when set to True, will perform a "full" vacuum and remove all files not referenced in the transaction log
+            full: when set to True, will perform a "full" vacuum and remove all files not referenced the transaction log.
+                when False, it will only vacuum not referenced files since last log checkpoint (or since genesis if no checkpoint exists).
             keep_versions: An optional list of versions to keep. If provided, files from these versions will not be deleted.
         Returns:
             the list of files no longer referenced by the Delta Table and are older than the retention threshold.