Revise AOP proxying section of the reference manual

[sbrannen]

Overview

The section on AOP proxying currently:

• omits some of the limitations of CGLIB-based proxies
• suggests the use of AopContext.currentProxy(), without mentioning the ability to make use of self injection instead
• uses flippant language such as "nothing-special-about-it", "absolutely horrendous", "gah!", etc.

We should therefore revise that section to improve those things.

Related Issues

• Recommend use of self injection instead of AopContext.currentProxy() #33449
• Consolidate self injection documentation in the reference manual #33454