[Python] Document the import-time default of PeriodicImpulse start_timestamp

sdks/python/apache_beam/io/fileio.py

@@ -404,7 +404,8 @@ def __init__(
       file_pattern: The file path to read from.
       interval: Interval at which to check for files in seconds.
       has_deduplication: Whether files already read are discarded or not.
-      start_timestamp: Timestamp for start file checking.
+      start_timestamp: Timestamp for start file checking. Defaults to the time
+        this module was first imported, not the time the transform is created.
       stop_timestamp: Timestamp after which no more files will be checked.
       match_updated_files: (When has_deduplication is set to True) whether match
         file with timestamp changes.

sdks/python/apache_beam/transforms/periodicsequence.py

@@ -299,7 +299,10 @@ def __init__(
       data: Optional[Sequence[Any]] = None,
       rebase: RebaseMode = RebaseMode.REBASE_NONE):
     '''
-    :param start_timestamp: Timestamp for first element.
+    :param start_timestamp: Timestamp for first element. Defaults to the time
+      this module was first imported, not the time the transform is created
+      or the pipeline starts. Pass an explicit timestamp, or use `rebase`, to
+      start relative to pipeline execution.
     :param stop_timestamp: Timestamp at or after which no elements will be
       output.
     :param fire_interval: Interval in seconds at which to output elements.