Add a note to the guide on ApplicationContext that accessing it from application code should be avoided

[odrotbohm]

The "Understanding ApplicationContext" document describes how to obtain an ApplicationContext but doesn't mention that this shouldn't actually be necessary from application code in most cases.

The casual reader – especially when being led there from a getting started guide – might not be deeply familiar with the concept of DI and thus get the impression, that this a common pattern.

[odrotbohm]

Awesome, thanks!