[docs] [issue31487] Improve f-strings documentation wrt format specifiers

[Mariatta Wijaya]

Changes by Mariatta Wijaya <mariatta.wijaya at gmail.com>:


----------
versions: +Python 3.6, Python 3.7

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31487>
_______________________________________