Update compatiblity-mode information around v0.31.0 (#240)

Co-authored-by: Ivan Mirić <[email protected]>
Co-authored-by: na-- <[email protected]>

Author: 3 people

src/data/markdown/docs/01 guides/02 Using k6/19 Javascript Compatibility Mode.md

@@ -117,10 +117,9 @@ $ K6_COMPATIBILITY_MODE=extended k6 run script.js
 
 </CodeGroup>
 
-Transforms scripts to ES5.1 using Babel with ES2015 preset, and loads
-[core.js](https://github.com/zloirock/core-js) v2 polyfills. This is
-the default mode, providing greater JavaScript compatibility, at the
-expense of startup time, RAM usage and performance.
+In case of syntax/parsing errors, the script will be transformed using Babel with specific plugins bringing the compatibility to ES2015(ES6)+. This means that features such as classes and arrow functions can be used. This does take some time to transpile and the produced code has slightly different line/column numbers. 
+
+Before k6 v0.31.0, [core.js](https://github.com/zloirock/core-js) v2 and even more Babel plugins were also included in extended mode. This added around 2MB extra memory usage per VU and some of the transformations (generators, async/await) of Babel were not useful as they were still not enough for k6 to just work with those features. So, before v0.31.0, using `--compatibility-mode=base` was a significant improvement on memory usage, which also translated to some CPU gains.
 
 ## Performance Comparison