[docs] [issue29677] clarify docs about 'round()' accepting a negative integer for ndigits

[Mariatta Wijaya]

Mariatta Wijaya added the comment:

Cheryl, thank you for your interest in working on this issue.

The original issue has been addressed in Gerrit Holl's PR. But it seems like the docs can be further improved.

Here is Victor's comment about it: (copied from GH-357)

```
while we are modifying the doc, I dislike the start: "Return the floating point value number": number can be an integer. round(123, -2) doesn't use floating point numbers. Maybe rephrase to following text?

Return number rounded to (...). number can be an integer or a floating point number.

Or just remove "floating pointer number"?
```

Will you be interested in preparing a PR to address Victor's comment above?

Thanks :)

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stage:  -> needs patch
versions:  -Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29677>
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