Docs for `CommandContext` should warn about task cancellation

[charypar]

The shell communication futures in CommandContext can only proceed when their associated RequestHandle is resolved. If the handle is droped, the task awaiting the future can never proceed, and the Command executor will evict it to avoid leaking memory.

This can lead to a surprising behaviour where tasks terminate unexpectedly. We should warn about this situation in the documentation for the methods which return futures susceptible to this behaviour.

[mhedgpeth]

Linking this to #352 as a possible place to fix this