Add examples of tricky page-turner usages not mentioned in the docs

[a1akris]

1. There is some confusion around pinning. Add a documented example on how to deal with not Unpin streams and explain why Unpin is not implemented by default. These snippets can be used as templates.

2. Besides handling pagination page turners can also be abused for some fun stuff because essentially they're complete async generators. Collect such examples of abuse but strongly discourage users to replicate them in production without approval of a team :)