docs: Clarify custom column keyword arguments in ecephys tutorial

[Copilot]

Fix #2093. Users are confused by label in nwbfile.add_electrode(label=...) thinking it's a built-in parameter, when it's actually a custom column defined earlier via add_electrode_column. The same ambiguity exists for quality in add_unit.

Changes

• docs/gallery/domain/ecephys.py: Added inline comments to add_electrode and add_unit calls clarifying that custom column values are passed as keyword arguments:

nwbfile.add_electrode(
    group=electrode_group,
    label="shank{}elec{}".format(ishank, ielec),  # custom column data can be added as keyword arguments
    location="brain area",
)

nwbfile.add_unit(spike_times=spike_times, quality="good")  # custom column data can be added as keyword arguments

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.10%. Comparing base (286de2d) to head (f207cbc).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2176   +/-   ##
=======================================
  Coverage   95.10%   95.10%           
=======================================
  Files          29       29           
  Lines        2943     2943           
  Branches      443      443           
=======================================
  Hits         2799     2799           
  Misses         86       86           
  Partials       58       58           

Flag	Coverage Δ
integration	72.85% <ø> (ø)
unit	85.35% <ø> (ø)

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