[3.7] Revert bpo-39576: docs: set context for decimal arbitrary precision arithmetic (GH-20746)

This reverts commit 00e4587.

Author: Stefan Krah

Doc/library/decimal.rst

@@ -2130,67 +2130,17 @@ Q. Is the CPython implementation fast for large numbers?
 A. Yes.  In the CPython and PyPy3 implementations, the C/CFFI versions of
 the decimal module integrate the high speed `libmpdec
 <https://www.bytereef.org/mpdecimal/doc/libmpdec/index.html>`_ library for
-arbitrary precision correctly-rounded decimal floating point arithmetic [#]_.
+arbitrary precision correctly-rounded decimal floating point arithmetic.
 ``libmpdec`` uses `Karatsuba multiplication
 <https://en.wikipedia.org/wiki/Karatsuba_algorithm>`_
 for medium-sized numbers and the `Number Theoretic Transform
 <https://en.wikipedia.org/wiki/Discrete_Fourier_transform_(general)#Number-theoretic_transform>`_
-for very large numbers.
+for very large numbers.  However, to realize this performance gain, the
+context needs to be set for unrounded calculations.
 
-The context must be adapted for exact arbitrary precision arithmetic. :attr:`Emin`
-and :attr:`Emax` should always be set to the maximum values, :attr:`clamp`
-should always be 0 (the default).  Setting :attr:`prec` requires some care.
+    >>> c = getcontext()
+    >>> c.prec = MAX_PREC
+    >>> c.Emax = MAX_EMAX
+    >>> c.Emin = MIN_EMIN
 
-The easiest approach for trying out bignum arithmetic is to use the maximum
-value for :attr:`prec` as well [#]_::
-
-    >>> setcontext(Context(prec=MAX_PREC, Emax=MAX_EMAX, Emin=MIN_EMIN))
-    >>> x = Decimal(2) ** 256
-    >>> x / 128
-    Decimal('904625697166532776746648320380374280103671755200316906558262375061821325312')
-
-
-For inexact results, :attr:`MAX_PREC` is far too large on 64-bit platforms and
-the available memory will be insufficient::
-
-   >>> Decimal(1) / 3
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in <module>
-   MemoryError
-
-On systems with overallocation (e.g. Linux), a more sophisticated approach is to
-adjust :attr:`prec` to the amount of available RAM.  Suppose that you have 8GB of
-RAM and expect 10 simultaneous operands using a maximum of 500MB each::
-
-   >>> import sys
-   >>>
-   >>> # Maximum number of digits for a single operand using 500MB in 8 byte words
-   >>> # with 19 (9 for the 32-bit version) digits per word:
-   >>> maxdigits = 19 * ((500 * 1024**2) // 8)
-   >>>
-   >>> # Check that this works:
-   >>> c = Context(prec=maxdigits, Emax=MAX_EMAX, Emin=MIN_EMIN)
-   >>> c.traps[Inexact] = True
-   >>> setcontext(c)
-   >>>
-   >>> # Fill the available precision with nines:
-   >>> x = Decimal(0).logical_invert() * 9
-   >>> sys.getsizeof(x)
-   524288112
-   >>> x + 2
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in <module>
-     decimal.Inexact: [<class 'decimal.Inexact'>]
-
-In general (and especially on systems without overallocation), it is recommended
-to estimate even tighter bounds and set the :attr:`Inexact` trap if all calculations
-are expected to be exact.
-
-
-.. [#]
-    .. versionadded:: 3.3
-
-.. [#]
-    .. versionchanged:: 3.9
-       This approach now works for all exact results except for non-integer powers.
-       Also backported to 3.7 and 3.8.
+.. versionadded:: 3.3