Document @Conditional gating of nested @Configuration classes

[Dennis-Mircea]

Summary

The "With nested @Configuration classes" section of the @Configuration Javadoc previously mentioned @Profile working with nested classes but was silent on how @Conditional interacts with them. The corresponding reference-doc section ("Conditionally Include @Configuration Classes or @Bean Methods" in composing-configuration-classes.adoc) was similarly silent.

In practice, whether an enclosing class's @Conditional gates a nested @Configuration depends on how the nested class is discovered. Nested classes reached through parser recursion from their enclosing class or via @Import inherit the enclosing's conditions through the existing importedBy mechanism (ConfigurationClassBeanDefinitionReader.TrackedConditionEvaluator.shouldSkip). Nested classes discovered independently (via @ComponentScan or by directly registering them) are processed using only their own conditions. The rule is real, long-standing, and currently undocumented.

This PR adds a paragraph to the @Configuration Javadoc and a [NOTE] admonition to the reference-doc page documenting that rule and recommending the redeclare-or-meta-annotation workaround when the same gating is intended on independent discovery paths.

See also #36820 for a separate proposal to close the discovery-path asymmetry at the framework level. This PR is independent of that proposal; it documents current behavior.

Test plan

• ./gradlew :spring-context:check (includes javadoc with -Werror, checkstyle, runtime-hints, tests), all green
• ./gradlew :framework-docs:antora, all green
• Locally rendered Antora site verified, admonition renders correctly under "Conditionally Include @Configuration Classes or @Bean Methods"
• Locally rendered Javadoc for @Configuration verified, new paragraph reads cleanly and {@link} references resolve

[sbrannen]

Thanks for the PR. I've requested a few changes.