DOC: Clarify process-noise input dimensions in LQE - #1256

Summary

Clarify the default process-noise input matrix and covariance dimensions for the lqe and dlqe estimators.

Addresses #1159 without changing the existing estimator behavior.

Changes

• Document that lqe(sys, QN, RN) uses sys.B as the process-noise input matrix G.
• Clarify that QN must have shape (sys.ninputs, sys.ninputs), which is not necessarily the same as the state dimension.
• Explain how to specify a different noise input matrix using the explicit matrix-based interface.
• Apply the same clarification to the discrete-time dlqe interface.
• Correct the documentation to identify StateSpace as the supported system input and replace incorrect dlqr references with dlqe.

Changes are limited to control/stochsys.py and doc/stochastic.rst.

No estimator algorithms, public APIs, or runtime behavior are modified.

Testing

All five CI workflows passed on the fork:

• Ruff
• Doctest
• Conda-based pytest
• Slycot from source
• Setup, Examples, and Notebooks

This is a documentation-only change. Existing tests passed without modification.

AI assistance was used for documentation drafting, source review, and validation.