docs: Warn against storing Number in static variables

A Number is normalized to the MantissaRange that is active when it is built. Comparisons between Numbers built under different ranges give wrong results. kAMMInvariantRelativeTolerance is one example: under the Small range it compares as about 1e-13, not 1e-11.

include/xrpl/basics/Number.h

@@ -280,6 +280,13 @@ concept Integral64 = std::is_same_v<T, std::int64_t> || std::is_same_v<T, std::u
  *   2. The max of the "large" range, 10^19-1, is the largest 10^X-1 value that
  *      fits in an unsigned 64-bit number. (10^19-1 < 2^64-1 and
  *      10^20-1 > 2^64-1). This avoids under- and overflows.
+ *   3. Do not store a Number in a static, inline, or global variable. A Number
+ *      is normalized to the MantissaRange that is active when it is built, and
+ *      operator< and operator== give wrong results when the two operands were
+ *      built under different ranges. Store the mantissa and exponent instead,
+ *      and build the Number where you use it. Changing a runtime Number into a
+ *      stored constant changes transaction results, so gate that change behind
+ *      an amendment.
  *
  * ---- External Interface ----
  *

include/xrpl/ledger/helpers/AMMHelpers.h

@@ -49,6 +49,8 @@ reduceOffer(auto const& amount)
 
 enum class IsDeposit : bool { No = false, Yes = true };
 
+// This constant is built at startup under the Large MantissaRange. Under the
+// Small range it compares as about 1e-13, not 1e-11. See note 3 on Number.
 inline Number const kAMMInvariantRelativeTolerance{1, -11};
 
 /**