fix eigfact doc

Backport of 969539267f6dc9d2cf
Document eigenvector format. Closes #8383

- Documentation for eigvecs and eigfact now explain that eigenvectors are stored in columns of F[:vectors].
- Also added output type annotations for eigvecs and eigfact.
- Minor language cleanup
- Add line breaks

Backport of 542cf9d93eaa95e4