In Mathematica, when you hover any a built-in function, a small window with 2 symbols appears (as in the third paragraph of this reference page, see picture): clicking on the former one gives you a quick description of the function ("summary"), whereas the latter is a link to a detailed page on the function ("documentation").

My question is if it's possible to do exactly the same on a function defined by the user.

_{(source: wolfram.com)}