Improve readability of Gas docs and add section on overflow potential

Author: webmaster128

docs/GAS.md

@@ -5,15 +5,19 @@ including CPU time and storage cost. It's unit is 1, i.e. you can think of it as
 countable points. Gas consumption is deterministic, so executing the same thing
 costs the same amount of gas across all hardware and operating systems.
 
+## CosmWasm gas vs. Cosmos SDK gas
+
 CosmWasm charges gas for Wasm operations, calls to host functions and calls to
-the Cosmos SDK. CosmWasm gas is different from Cosmos SDK gas as the numbers
+the Cosmos SDK. _CosmWasm gas_ is different from _Cosmos SDK gas_ as the numbers
 here are much larger. Since we charge gas for arbitrary user defined operations,
 we need to charge each Wasm operation individually and cannot group larger tasks
 together. As a result, the gas values become much larger than in Cosmos SDK even
 for very fast executions. There is a [multiplier][defaultgasmultiplier] to
 translate between CosmWasm gas and Cosmos SDK. It was measured and set to 100 a
 while ago and can be adjusted when necessary.
 
+## CosmWasm gas pricing
+
 For CosmWasm gas, the target gas consumption is 1 Teragas (10^12 gas) per
 millisecond. This idea is [inspired by NEAR][neargas] and we encourage you to
 read their excellent docs on that topic.
@@ -38,3 +42,26 @@ multiple ways:
   https://github.com/CosmWasm/wasmd/blob/v0.19.0/x/wasm/keeper/gas_register.go#L18
 [neargas]: https://docs.near.org/docs/concepts/gas
 [#1120]: https://github.com/CosmWasm/cosmwasm/pull/1120
+
+## Gas overflow potential
+
+CosmWasm gas aims for 1 Teragas/second, i.e. the uint64 range exceeds after 18
+billion seconds (213 days)<sup>1</sup>. Assuming a max supported block execution
+time of 100 seconds, the gas price has to be over-priced by a factor of 184
+million (184 Petagas/second) in order to exceed the uint64 range<sup>2</sup>.
+Since serious over or underpricing is considered a bug, using uint64 for gas
+mesurments is considered safe.
+
+Cosmos SDK gas uses values that are smaller by a factor of 150_000, so those
+don't overflow as well. Since no Cosmos SDK gas values are processed inside of
+this repository, this is not our main concern. However, it's good to know that
+we can safely pass them in uint64 fields, as long as the full range is
+supported. This is the case for the C API as well as
+[JSON numbers](https://www.json.org/) as long as both sides support integers in
+their JSON implementation. Go and Rust do that while many other implementations
+don't support integers numbers and convert to them to IEEE-754 doubles, which
+has a safe integer range up about 53 bit (e.g. JavaScript and jq).
+
+<sup>1</sup> Python3: `(2**64-1) / 10**12`
+
+<sup>2</sup> Python3: `((2**64-1)/100) / 10**12`