07-cond.md: Introduce abs, linking to documentation (#841)

* Introduce `abs`, linking to documentation

* Hide abs hint and exemplify usage

Author: ldko

_episodes/07-cond.md

@@ -308,6 +308,19 @@ freeing us from having to manually examine every plot for features we've seen be
 > Compare your implementation with your partner's:
 > do you get the same answer for all possible pairs of numbers?
 >
+> > ## Hint
+> > There is a [built-in function `abs`][abs-function] that returns the absolute value of
+> > a number:
+> > ~~~
+> > print(abs(-12))
+> > ~~~
+> > {: .language-python}
+> > ~~~
+> > 12
+> > ~~~
+> > {: .output}
+> {: .solution}
+>
 > > ## Solution 1
 > > ~~~
 > > a = 5
@@ -477,4 +490,6 @@ freeing us from having to manually examine every plot for features we've seen be
 > {: .solution}
 {: .challenge}
 
+[abs-function]: https://docs.python.org/3/library/functions.html#abs
+
 {% include links.md %}