Update date and add missing entries in release notes for 2.3.0 (#474)

Author: rly

docs/format/source/format_release_notes.rst

@@ -1,15 +1,18 @@
 Release Notes
 =============
 
-2.3.0 (April 23, 2021)
+2.3.0 (May 11, 2021)
 ----------------
 
 - Add optional ``waveforms`` column to the ``Units`` table.
 - Add optional ``strain`` field to ``Subject``.
 - Add to ``DecompositionSeries`` an optional ``DynamicTableRegion`` called ``source_channels``.
 - Add to ``ImageSeries`` an optional link to ``Device``.
 - Add optional ``continuity`` field to ``TimeSeries``.
+- Add optional ``filtering`` attribute to ``ElectricalSeries``.
 - Clarify documentation for electrode impedance and filtering.
+- Add description of extra fields.
+- Set the ``stimulus_description`` for ``IZeroCurrentClamp`` to have the fixed value ``N/A``.
 - Update hdmf-common-schema from 1.1.3 to version 1.5.0.
   - The HDMF-experimental namespace was added, which includes the ``ExternalResources`` and ``EnumData``
     data types. Schema in the HDMF-experimental namespace are experimental and subject to breaking changes at any time.