Spectral layout docstring

[karlmoresco]

Description

Updated the spectral_layout docstring with details and information about arguments, returns, raises etc.

PR checklist:

• This comment contains a description of changes
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have checked my code and documentation and corrected any misspellings

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Note

Low Risk
Docstring-only change with no behavioral impact on layout computation or callers.

Overview
Expands the spectral_layout docstring in _networkx.py from a single-line description to full API documentation.

The update explains how spectral layout uses Laplacian eigenvectors, documents g, dim, normalize, seed, and method (eigen vs psvd), and describes the returned node→coordinate mapping. It also lists ValueError cases that match existing validation (graph size/connectivity, dim, method/normalize pairing, and method-specific size or bipartite/pixel_type constraints).

No runtime or algorithm changes—documentation only.

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