Add docs for Callback config

[kjbracey]

Documentation for changes in ARMmbed/mbed-os#12036

[40Grit]

@AGlass0fMilk

[AnotherButler]

@iriark01 This is waiting on engineering review before editing.

[kjbracey]

Part of the change was moved to ARMmbed/mbed-os#12761. These docs are largely fine if that doesn't make it, but the wording does strongly suggest that the default for the callback-nontrivial is false. It's true until 12761 lands.

[iriark01]

@kjbracey-arm was the intention to hold on this PR until 12761 makes it in?

[kjbracey]

I'm fine with it going now, but maybe it could be edited. That first paragraph talking about how the "setting can almost always be left set to false" implies that it is false by default, but it isn't yet.

Maybe just take out the word "left"? Then it works either way, I think.

[iriark01]

@kjbracey-arm looks like I can merge this?

[kjbracey]

Yes, thanks!