bpo-23460: Fix documentation for decimal string :g formatting (GH-11850)

Doc/library/string.rst

@@ -500,9 +500,10 @@ The available presentation types for floating point and decimal values are:
    |         |                                                          |
    |         | The precise rules are as follows: suppose that the       |
    |         | result formatted with presentation type ``'e'`` and      |
-   |         | precision ``p-1`` would have exponent ``exp``.  Then     |
-   |         | if ``-4 <= exp < p``, the number is formatted            |
-   |         | with presentation type ``'f'`` and precision             |
+   |         | precision ``p-1`` would have exponent ``exp``.  Then,    |
+   |         | if ``m <= exp < p``, where ``m`` is -4 for floats and -6 |
+   |         | for :class:`Decimals <decimal.Decimal>`, the number is   |
+   |         | formatted with presentation type ``'f'`` and precision   |
    |         | ``p-1-exp``.  Otherwise, the number is formatted         |
    |         | with presentation type ``'e'`` and precision ``p-1``.    |
    |         | In both cases insignificant trailing zeros are removed   |

Misc/NEWS.d/next/Documentation/2019-02-14-07-12-48.bpo-23460.Iqiqtm.rst

+The documentation for decimal string formatting using the `:g` specifier has been updated to reflect the correct exponential notation cutoff point. Original patch contributed by Tuomas Suutari.
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