DOC: stats: Extend documentation of random_correlation matrix

[vrossum]

DOC: _multivariate.py
Extended documentation of random_correlation matrix.

• state conditions on eig input.
• describe properties of output

[melissawm]

Hi @vrossum !

• If you are addressing an existing issue, please add the expression Closes #<issue number> to your PR description.
• If you can, please make sure you use our [commit message conventions] and, for this case, you can also use [docs only] at the end of your commit message to make sure we don't spend CI minutes unnecessarily. (see https://docs.scipy.org/doc/scipy/dev/contributor/continuous_integration.html#skipping for more information)

EDIT: I also just noticed you made your changes from your main branch. This is generally not a good idea - we recommend always using feature branches for PRs. See https://docs.scipy.org/doc/scipy/dev/contributor/development_workflow.html#basic-workflow

Thank you!

[j-bowhay]

Thanks for your first contribution to SciPy, keep them coming!