[docs] [issue29928] Add f-strings to Glossary

[Mariatta Wijaya]

Mariatta Wijaya added the comment:

The abstract of PEP 498 states:
```
In this PEP, such strings will be referred to as "f-strings", taken from the leading character used to denote such strings, and standing for "formatted strings".
```

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29928>
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