Prior to this commit, it was unclear to users and third parties that it
is necessary to manually configure a StandardTypeLocator with a
specific ClassLoader to ensure that the SpEL expression parser is able
to reliably locate user types.
For example, the StandardBeanExpressionResolver in the spring-context
module configures a StandardTypeLocator using the bean ClassLoader of
the corresponding BeanFactory.
This commit improves the documentation to raise awareness of this fact.
Closes gh-26253
10de295a72