Add documentation for custom type support

When a type has a constructor that takes a string, it can be used for options. I could not find this documented in the wiki. The exception message that provides a hint for that is unfortunately also swallowed.

Start by reviewing the wiki’s options and type-conversion documentation alongside the custom type support described in the issue. Confirm how a string constructor enables a type to be used for options and how the exception hint is presented, then document the supported behavior with an example. Done means the wiki clearly explains this path and its limitations.

Written by the indexing model from the issue text.