Make JavadocEmptyLineBeforeTagCheck messages self-describing (#1563).

The two violations from this check now name the triggering condition
(single paragraph vs. multiple paragraphs) so they no longer read as
contradictions and a user can mechanically tell which fix to apply.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yegor256

src/main/java/com/qulice/checkstyle/JavadocEmptyLineBeforeTagCheck.java

@@ -114,12 +114,12 @@ private void inspect(final String[] lines, final int start, final int tag) {
             if (multi && !empty) {
                 this.log(
                     tag + 1,
-                    "Empty Javadoc line required before at-clauses"
+                    "Empty Javadoc line required before at-clauses, since the description has multiple paragraphs"
                 );
             } else if (!multi && empty) {
                 this.log(
                     tag,
-                    "Empty Javadoc line before at-clauses is not allowed"
+                    "Empty Javadoc line before at-clauses is not allowed, since the description is a single paragraph"
                 );
             }
         }

src/test/resources/com/qulice/checkstyle/ChecksTest/JavadocEmptyLineBeforeTagCheck/violations.txt

@@ -1,5 +1,5 @@
-8:Empty Javadoc line before at-clauses is not allowed
-15:Empty Javadoc line before at-clauses is not allowed
-24:Empty Javadoc line required before at-clauses
-31:Empty Javadoc line before at-clauses is not allowed
-43:Empty Javadoc line required before at-clauses
+8:Empty Javadoc line before at-clauses is not allowed, since the description is a single paragraph
+15:Empty Javadoc line before at-clauses is not allowed, since the description is a single paragraph
+24:Empty Javadoc line required before at-clauses, since the description has multiple paragraphs
+31:Empty Javadoc line before at-clauses is not allowed, since the description is a single paragraph
+43:Empty Javadoc line required before at-clauses, since the description has multiple paragraphs