Every function of an interface demands consideration concerning how users will be instructed on how to find and use it.
Certain features, like Android's push-shift-twice-for-caps-lock on the physical keyboard, aren't explicitly mentioned anywhere in the UI itself. (Maybe the instruction manual, but who reads those?) The feature is discovered almost assuredly by accident when a user tries to toggle Shift off, and finds it in Caps Lock mode. Because stumbling upon this particular feature doesn't really do any major damage, relying on accidental discovery seems effective enough in this scenario, and users will remember how to use it from that point on.
There are other ways of alerting a user to a feature, but I don't see where such instructions would be useful here. When a user pushes Shift, a message might inform them of the Caps Lock function, or the message could appear every time until the user finally uses it or tells the phone to stop reminding them. Either way, such messages take real estate, and I imagine they'd be much more annoying than helpful.