Eliminate all Javadoc warnings

- Support external Javadoc links using Gradle's javadoc.options.links

 - Fix all other Javadoc warnings, such as typos, references to
   non-existent (or no longer existent) types and members, etc,
   including changes related to the Quartz 2.0 upgrade (SPR-8275) and
   adding the HTTP PATCH method (SPR-7985).

 - Suppress all output for project-level `javadoc` tasks in order to
   hide false-negative warnings about cross-module @see and @link
   references (e.g. spring-core having a @see reference to spring-web).
   Use the `--info` (-i) flag to gradle at any time to see project-level
   javadoc warnings without running the entire `api` task. e.g.
   `gradle :spring-core:javadoc -i`

 - Favor root project level `api` task for detection of legitimate
   Javadoc warnings. There are now zero Javadoc warnings across the
   entirety of spring-framework. Goal: keep it that way.

 - Remove all @link and @see references to types and members that exist
   only in Servlet <= 2.5 and Hibernate <= 4.0, favoring 3.0+ and 4.0+
   respectively. This is necessary because only one version of each of
   these dependencies can be present on the global `api` javadoc task's
   classpath. To that end, the `api` task classpath has now been
   customized to ensure that the Servlet 3 API and Hibernate Core 4 jars
   have precedence.

 - SPR-8896 replaced our dependency on aspectjrt with a dependency on
   aspectjweaver, which is fine from a POM point of view, but causes
   a spurious warning to be emitted from the ant iajc task that it
   "cannot find aspectjrt on the classpath" - even though aspectjweaver
   is perfectly sufficient. In the name of keeping the console quiet, a
   new `rt` configuration has been added, and aspectjrt added as a
   dependency to it. In turn, configurations.rt.asPath is appended to
   the iajc classpath during both compileJava and compileTestJava for
   spring-aspects.

Issue: SPR-10078, SPR-8275, SPR-7985, SPR-8896

spring-beans/src/main/java/org/springframework/beans/factory/config/PropertyPlaceholderConfigurer.java

@@ -37,8 +37,8 @@ import org.springframework.util.StringValueResolver;
  *
  * <p>{@link PropertyPlaceholderConfigurer} is still appropriate for use when:
  * <ul>
- * <li>the {@link org.springframework.context spring-context} module is not available (i.e., one is using
- * Spring's {@code BeanFactory} API as opposed to {@code ApplicationContext}).
+ * <li>the {@code spring-context} module is not available (i.e., one is using Spring's
+ * {@code BeanFactory} API as opposed to {@code ApplicationContext}).
  * <li>existing configuration makes use of the {@link #setSystemPropertiesMode(int) "systemPropertiesMode"} and/or
  * {@link #setSystemPropertiesModeName(String) "systemPropertiesModeName"} properties. Users are encouraged to move
  * away from using these settings, and rather configure property source search order through the container's