feat: update the example and comment about \varnothing

[Myriad-Dreamin]

I updated the example and comment about \varnothing.

• To keep the output in 2 pages, I had to cut down the sentences, resulted in some loss of meaning.
• It's important to note that copying text from a PDF may not give you the exact same Unicode characters, since the result also depends on the algorithm used for mapping character codes to Unicode values, as explained in Stack Overflow.
  • Nevertheless, I added a note that according to newcm-doc, the Unicode code used for \varnothing is possible U+2300.

See related issue #10.

[johanvx]

Nice work! Thanks a lot!