Article: Python abs() (freeCodeCamp#825)

* Add python-abs.md

* Fix built-in

* Add some fixes

* Rename python abs

Python-Function-ABS.md

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+# Python `abs(x)`
+
+`abs()` is a built-in function in Python 3, to compute the absolute value of any number. It takes one argument `x`. The argument can even be a [complex number](https://docs.python.org/3.0/library/cmath.html), and in that case its [modulus](http://www.mathcentre.ac.uk/resources/sigma%20complex%20number%20leaflets/sigma-complex9-2009-1.pdf) is returned.
+
+## Argument
+It takes one argument `x` - an integer, or decimal, or a complex number.
+
+## Return Value
+The return value would be a positive number. Even if complex number is passed, it would return its magnitude, computed as per complex number algebra.
+
+## Code Sample
+
+```python
+print(abs(3.4)) # prints 3.4
+print(abs(-6)) # prints 6
+print(abs(3 + 4j)) # prints 5, because |3 + 4j| = 5
+```
+:rocket: [REPL It!](https://repl.it/CL8k/0)
+
+[Official Docs](https://docs.python.org/3/library/functions.html#abs)