Add docstring to ParametricUMAP placeholder class for better documentation

[Lang-Qiu]

Problem

The ParametricUMAP class in umap/__init__.py, which serves as a placeholder when TensorFlow is not installed, was missing a docstring. This reduced code readability and made it harder for users to understand its purpose and error handling.

Solution

Added a comprehensive docstring to the ParametricUMAP class. The docstring includes:

• A clear description of the class's role as a placeholder when TensorFlow is unavailable.
• Usage examples to help users handle import scenarios gracefully.
• References to related functions (e.g., load_ParametricUMAP) for better discoverability.

Verification

• Review the updated __init__.py file to confirm the docstring is present and correctly formatted.
• Run existing tests to ensure no functional changes or regressions (e.g., python -m pytest in the repository).
• Check that the class's behavior remains unchanged: it should still warn and raise an ImportError when instantiated without TensorFlow.