Update: I think the more general question I should have asked is: How should I write usage messages to work well with auto-completion? Is there documentation on how to prepare a package for convenient auto-completion?
If I make a small function with a usage message, Mathematica will automatically extract the argument pattern from the usage message and offer it for auto-completion:
f[] := 1
f::usage = "f[] returns the value.";

Notice that the complete usage message is echoed under the completion patters, `"f[] returns the value.", including "f[]".
This is not the case for builtins. The argument pattern is not repeated in the description. Can I achieve this for user-defined functions?

f, the autocompletion doesn't take it into account, even if I quit the kernel and enter the same new usage message again ! You have to quit mma and start a new session ! – SquareOne Oct 06 '15 at 12:59f[]" part isn't removed from the second line. http://i.stack.imgur.com/arY1E.gif – Karsten7 Oct 06 '15 at 13:00FileNameJoin[{$InstallationDirectory, "SystemFiles", "Kernel", "TextResources", "English"}]. Especially FunctionInformation2.m seems to contain the informations for the auto-completion floating box. – Karsten7 Oct 24 '15 at 01:01