Document Mem.pin more explicitly to make it more findable

[david-macmahon]

The concept of memory pinning is discussed in the “Tasks and threads” section of the documentation (in multitasking.md), but Mem.pin is only shown as part of a larger example. It would nice to have Mem.pin documented in the “CUDA driver” section (driver.md) alongside alloc, register, advise, etc. A doc string would be nice too.