gh-128185: Align Decimal docs with spec (case irrelevant for nan/inf) (#128323)

Author: skirpichev

Doc/library/decimal.rst

@@ -367,6 +367,8 @@ Decimal objects
    appears above.  These include decimal digits from various other
    alphabets (for example, Arabic-Indic and Devanāgarī digits) along
    with the fullwidth digits ``'\uff10'`` through ``'\uff19'``.
+   Case is not significant, so, for example, ``inf``, ``Inf``, ``INFINITY``,
+   and ``iNfINity`` are all acceptable spellings for positive infinity.
 
    If *value* is a :class:`tuple`, it should have three components, a sign
    (``0`` for positive or ``1`` for negative), a :class:`tuple` of