document `delayed`

[shashi]

Just to think over what would happen if users start to use delayed a la carte:

1. mapvalues -- returning delayed(f)(x) is completely OK. The chaining of lazy values should work as expected. Calling delayed functions with other values which may themselves be delayed is OK. But calling it with say an array of delayed values is not.
2. mapsubtrees -- returning delayed(f)(x) will work but not when f(x) is a FileTree. However, a FileTree with delayed values is OK.
3. reducevalues -- seems fine again. as in 1.

cc @DrChainsaw

[shashi]

Maybe users want to apply a function such that

• if any of the inputs are lazy, make the output lazy
• if any none of them are lazy, just compute it.

This is done internally by FileTrees.lazy_if_lazy(f) but I don't know if we should just export that.