Update metadata docstrings

[sjvenditto]

Summary

This PR updates the docstrings for new metadata functions, as well as docstrings for classes that inherit metadata.

• all functions and properties in metadata_class.py, notably docstring examples added to set_info and get_info methods
• interval_set.py > IntervalSet, time_series.py > TsdFrame, and ts_group.py > TsGroup: doc strings that were previously split under class definition and init have been merged, and new docstring examples added for initializing each class with and without metadata
• docstrings were added to some properties throughout IntervalSet, TsGroup, TsdFrame, _BaseTsd, and _Base
• "Metadata" section on the "User Guide > Core methods" documentation page has been updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
pynapple/core/base_class.py	99.11% <100.00%> (+0.02%)	⬆️
pynapple/core/interval_set.py	93.62% <100.00%> (+0.30%)	⬆️
pynapple/core/metadata_class.py	81.65% <100.00%> (+1.85%)	⬆️
pynapple/core/time_series.py	89.47% <100.00%> (+0.17%)	⬆️
pynapple/core/ts_group.py	92.26% <100.00%> (+0.24%)	⬆️

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