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.

(cherry picked from commit a46779d866b1b121adc73164215ba6437f53c208)