Add some member variables documentation

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/timeseries/_resampling.py

@@ -346,8 +346,13 @@ def __init__(self, config: ResamplerConfig) -> None:
             config: The configuration for the resampler.
         """
         self._config = config
+        """The configuration for this resampler."""
+
         self._resamplers: dict[Source, _StreamingHelper] = {}
+        """A mapping between sources and the streaming helper handling that source."""
+
         self._timer: Timer = Timer(config.resampling_period_s)
+        """The timer to trigger the next resampling."""
 
     @property
     def config(self) -> ResamplerConfig: