detect/transform: Clarify transformation validation

detect/transform: Clarify transformation validation

Issue: 6439

Clarify the transform validation step. When a transform indicates that
the content/byte-array is not compatible, validation will stop.

Content is incompatible is some cases -- e.g., following the
to_lowercase transform with content containing uppercase characters.
An alert is not possible since the content contains uppercase and the
transform has converted the buffer into all lowercase.