scripts: update docs for 'generate-unicode-tables'
scripts: update docs for 'generate-unicode-tables'

The docs are now updated to work with Unicode 14. (In particular,
emoji-data.txt no longer needs to be downloaded separately.) We also
include a note about adding a new case for "age" in regex-syntax.

rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex