Document appropriate implementation of `GetHashCode`.

VS and Rider both flag warnings when you use a property in a `GetHashCode` implementation that is not read-only, however, I recently encountered an issue where one of the properties was virtual. This allowed the property to change in a derived class and alter the base class hash code leading to a failed dictionary lookup.

Solution: flag as warning when virtual member is used in `GetHashCode`

No files or tests are named in the issue. Locate the repository's existing implementation for warnings involving GetHashCode, then inspect how member mutability and virtual members are analyzed; done means a virtual member used by GetHashCode produces a warning without breaking existing behavior.

Written by the indexing model from the issue text.