Warn about BigDecimal limitations (#130)

Author: dwardu

pages/en/developer/assemblyscript-api.mdx

@@ -93,6 +93,8 @@ import { BigDecimal } from '@graphprotocol/graph-ts'
 
 `BigDecimal` is used to represent arbitrary precision decimals.
 
+> Note: [Internally](https://github.com/graphprotocol/graph-node/blob/master/graph/src/data/store/scalar.rs) `BigDecimal` is stored in [IEEE-754 decimal128 floating-point format](https://en.wikipedia.org/wiki/Decimal128_floating-point_format), which supports 34 decimal digits of significand. This makes `BigDecimal` unsuitable for representing fixed-point types that can span wider than 34 digits, such as a Solidity [`ufixed256x18`](https://docs.soliditylang.org/en/latest/types.html#fixed-point-numbers) or equivalent.
+
 _Construction_
 
 - `constructor(bigInt: BigInt)` – creates a `BigDecimal` from an `BigInt`.