Added documentation for methods on shared types

[paandahl]

I only realized methods on shared types were possible by looking through the source code, so I thought it would be a good idea to add some documentation.

I added two sections with code snippets to the "Shared types" page:

• Methods defined in Rust
• Methods defined in C++

[ttzytt]

I'm new to the project and also looking for this. It would be really helpful if it is in the doc.

[ttzytt]

Wondering if it is possible to have methods implementing traits on shared types.