improve dependency version explanation: it's a requirement, not always effective version

currently, POM documentation says for dependency version https://maven.apache.org/ref/3.9.9/maven-model/maven.html#class_dependency
"The version of the dependency, e.g. 3.2.1. In Maven 2, this can also be specified as a range of versions."

users don't get that it is a version requirement only: the effective version will be resolved based on usage context, as explained more in details in https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Transitive_Dependencies

it's long overdue to clarify at POM level