Document the need to set an explicit delimiter when using ModificationBehavior::{Append,Prepend}

[edmorley]

Currently the upstream CNB spec doesn't actually define what the default delimiter should be, when using the append/prepend functionality of CNB env vars:
https://github.com/buildpacks/spec/blob/main/buildpack.md#delimiter

In practice, the lifecycle implementation defaults to an empty string delimiter, which is not helpful in most cases.

Resolving this confusing default behaviour is the subject of:
buildpacks/spec#285

It also affected the examples in this repo:
#398

In the meantime we should document the current behaviour in libcnb.rs LayerEnv docs:
https://docs.rs/libcnb/latest/libcnb/layer_env/struct.LayerEnv.html
https://docs.rs/libcnb/latest/libcnb/layer_env/enum.ModificationBehavior.html