gh-73487: Convert _decimal to use Argument Clinic (part 6)

[skirpichev]

• Issue: decimal: Use Argument Clinic #73487

[serhiy-storchaka]

LGTM. 👍

[skirpichev]

Docstrings diff:

--- ref.txt 2025-08-22 11:00:56.325746547 +0300
+++ patch.txt   2025-08-22 10:49:32.335072600 +0300
@@ -95,6 +95,8 @@
      |      traps=None
      |  )
      |
+     |  Create context.
+     |
      |  The context affects almost all operations and controls rounding,
      |  Over/Underflow, raising of exceptions and much more.  A new context
      |  can be constructed as follows:
@@ -182,12 +184,16 @@
      |      Copy the sign from y to x.
      |
      |  create_decimal(self, num='0', /)
-     |      Create a new Decimal instance from num, using self as the context. Unlike the
-     |      Decimal constructor, this function observes the context limits.
+     |      Create a new Decimal instance from num, using self as the context.
+     |
+     |      Unlike the Decimal constructor, this function observes the context
+     |      limits.
      |
      |  create_decimal_from_float(self, f, /)
-     |      Create a new Decimal instance from float f.  Unlike the Decimal.from_float()
-     |      class method, this function observes the context limits.
+     |      Create a new Decimal instance from float f.
+     |
+     |      Unlike the Decimal.from_float() class method, this function observes
+     |      the context limits.
      |
      |  divide(self, x, y, /)
      |      Return x divided by y.