Docs: Describe usage of non-ASCII characters in the sheet names

[sequba]

Page URL

https://hyperformula.handsontable.com/guide/cell-references.html

Improvement description

In HF only some characters are allowed in unqoted sheet names:

[A-Za-z\u00C0-\u02AF0-9_]

If a sheet name contains other characters in must be enclosed in single quotes. E.g.:

=IF('あは'!A2  > 'あは'!A3, "TeamA", "TeamB")

Related: https://github.com/handsontable/hyperformula/issues/1312