DOCS: improving "to_excel" function description. Explained more about engines
The basic description looked a bit confusing and did not explain why one engine should be used over the other. Also it didn't mention that both need to be installed and which was the default option.  I had 'openpyxl' installed in a previous project, and every time the file was 2x the normal size compared to using 'xlsxwriter' or opening and closing in Excel directly (manually). 
Feel free to reopen when you have time to circle back here