Add documentation for reserveDOMCache option

[evilaliv3]

By looking at the default options recommended i've found the reserveDOMCache option and i just noticed that it is not documented.

This ticket is to kindly ask for a clarification about this functionality and when it is recommended to enable it so that we could work on improving the documentation: https://github.com/gruntjs/grunt-contrib-uglify/blob/main/docs/uglify-options.md

Thank you!

[alexlamsl]

I believe that's a typo − it should be reserveDOMProperties:
https://github.com/gruntjs/grunt-contrib-uglify/blob/v5.1.0/README.md#reservedomproperties

Feel free to submit a Pull Request to fix the documentation.