Improve how the build deals with javadoc invalid references

This commit improves how the build deals with javadoc invalid references
in two ways.

Link/see references that are temporarily invalid during javadoc
generation of individual modules are better masked by using the option
`Xdoclint:syntax` instead of `Xdoclint:none` (warnings were still
visible in some cases, e.g. when individually building the javadoc for
a specific module).

Global javadoc-building task `api` now combines `syntax` and `reference`
`Xdoclint` groups, allowing to raise truly invalid references even when
all the modules have been aggregated.

This commit also fixes the 20+ errors which appeared following the later
change in doclet configuration.

Closes gh-30428

spring-tx/src/main/java/org/springframework/transaction/annotation/Isolation.java

@@ -70,7 +70,7 @@ public enum Isolation {
 	/**
 	 * A constant indicating that dirty reads, non-repeatable reads, and phantom
 	 * reads are prevented.
-	 * <p>This level includes the prohibitions in {@link #ISOLATION_REPEATABLE_READ}
+	 * <p>This level includes the prohibitions in {@link #REPEATABLE_READ}
 	 * and further prohibits the situation where one transaction reads all rows that
 	 * satisfy a {@code WHERE} condition, a second transaction inserts a row
 	 * that satisfies that {@code WHERE} condition, and the first transaction