Issue B4: Add Docstrings to Public Functions

[Ramdam17]

Description:

Improve documentation by adding or improving docstrings for 3-5 public functions in HyPyP.

Context:
Good docstrings help users understand what functions do without reading the source code. They also power IDE autocomplete and help systems.

Target functions (pick 3-5):

• analyses.pow() — Power spectral density
• analyses.compute_freq_bands() — Frequency band extraction
• analyses.compute_sync() — Synchrony metrics
• stats.statscond() — Statistical testing
• viz.viz_2D_topomap() — 2D visualization

Docstring format (NumPy style):

def function_name(param1, param2):
    """
    Short description of what the function does.

    Parameters
    ----------
    param1 : type
        Description of param1.
    param2 : type
        Description of param2.

    Returns
    -------
    type
        Description of return value.

    Examples
    --------
    >>> result = function_name(x, y)
    """

Acceptance Criteria:

• 3-5 functions have complete docstrings
• Docstrings follow NumPy format
• Each docstring includes at least one example
• Examples are tested and work correctly

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