Clean up issued currency format description

I realized that, despite the changes from #481, this section still had a typo ("significant digits and sig") in discussing the case for zero, and there was some redundancy as well.

I think the cleaned-up version should more clearly distinguish between the math and serialization format (so that the serialization format stuff can later be moved out to the serialization page)

content/references/rippled-api/api-conventions/currency-formats.md

@@ -31,18 +31,15 @@ Issued currencies in the XRP Ledger are represented with a custom format with th
 
 ![Issued Currency Amount Format diagram](img/currency-number-format.png)
 
-Internally, `rippled` represents numbers for issued currencies in a custom number format. This format can store a wide variety of assets, including those typically measured in very small or very large denominations. Unlike typical floating-point representations of non-whole numbers, this format uses integer math for all calculations, so it always maintains 15 decimal digits of precision. Unlike "arbitrary precision" number formats, the custom format can always be stored in a fixed size of 64 bits.
+Internally, `rippled` represents numbers for issued currencies in a custom number format. This format can store a wide variety of assets, including those typically measured in very small or very large denominations. This format uses significant digits and a power-of-ten exponent in a similar way to scientific notation. The format supports positive and negative significant digits and exponents within the specified range. Unlike typical floating-point representations of non-whole numbers, this format uses integer math for all calculations, so it always maintains 15 decimal digits of precision. The internal calculations generally use integer math so that numbers are always precise within 15 digits. Multiplication and division have adjustments to compensate for over-rounding in the least significant digits.
 
-The internal format consists of four parts: a "not XRP" bit, a sign bit, significant digits, and an exponent. (It uses them in the same way as scientific notation.) They are present in order:
+Unlike "arbitrary precision" number formats, the custom format can be stored in a fixed size of 64 bits. When serialized this way, the format consists of a "not XRP" bit, a sign bit, significant digits, and an exponent. They are present in order:
 
 1. The first (most significant) bit for an issued currency amount is `1` to indicate that it is not an XRP amount. (XRP amounts always have the most significant bit set to `0` to distinguish them from this format.)
 2. The sign bit indicates whether the amount is positive or negative. Unlike standard [two's complement](https://en.wikipedia.org/wiki/Two%27s_complement) integers, `1` indicates **positive** in the XRP Ledger format, and `0` indicates negative.
 3. The next 8 bits represent the exponent as an unsigned integer. The exponent indicates the scale (what power of 10 the significant digits should be multiplied by) in the range -96 to +80 (inclusive). However, when serializing, we add 97 to the exponent to make it possible to serialize as an unsigned integer. Thus, a serialized value of `1` indicates an exponent of `-96`, a serialized value of `177` indicates an exponent of 80, and so on.
-4. The remaining 54 bits represent the significant digits as an unsigned integer. When serializing, this value is normalized to the range 10<sup>15</sup> (`1000000000000000`) to 10<sup>16</sup>-1 (`9999999999999999`) inclusive, except for the special case of the value 0, whose significant digits and sig use the value `0`.  
+4. The remaining 54 bits represent the significant digits as an unsigned integer. When serializing, this value is normalized to the range 10<sup>15</sup> (`1000000000000000`) to 10<sup>16</sup>-1 (`9999999999999999`) inclusive, except for the special case of the value 0. There is a special case for the value 0. In this case, the sign bit, exponent, and mantissa are all zeroes, so the 64-bit value is serialized as `0x8000000000000000000000000000000000000000`.
 
-Before recording any amount, `rippled` "canonicalizes" the value so that the significant digits and exponent are within the expected range. For example, the canonical representation of 1 unit of currency is `1000000000000000e-15`. The internal calculations generally use integer math so that numbers are always precise within 15 digits. Multiplication and division have adjustments to compensate for over-rounding in the least significant digits.
-
-There is a special case for the value 0. In this case, the sign bit, exponent, and mantissa are all zeroes, so the 64-bit value is serialized as `0x8000000000000000000000000000000000000000`.
 
 ## Currency Codes