clarify goal vs phase terms in output and documentation

$ mvn release:prepare -DdryRun
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------------< groupId:artifactId >---------------
[INFO] Building Name 1.4.2-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-release-plugin:3.0.0-M5:prepare (default-cli) @ artifactId ---
[INFO] phase verify-release-configuration (dry-run)
[INFO] starting prepare goal in dry-run mode, composed of 17 phases: check-poms, scm-check-modifications, check-dependency-snapshots, create-backup-poms, map-release-versions, input-variables, map-development-versions, rewrite-poms-for-release, generate-release-poms, run-preparation-goals, scm-commit-release, scm-tag, rewrite-poms-for-development, remove-release-poms, run-completion-goals, scm-commit-development, end-release

$ mvn release:perform -DdryRun
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------------< groupId:artifactId >---------------
[INFO] Building Name 1.4.2-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-release-plugin:3.0.0-M5:perform (default-cli) @ artifactId ---
[INFO] phase verify-release-configuration (dry-run)
[INFO] starting perform goal in dry-run mode, composed of 3 phases: verify-completed-prepare-phases, checkout-project-from-scm, run-perform-goals