Rules in character sets undocumented

[codesections]

The docs describe how you can combine enumerated character sets (e.g., [0..9]) and/or Unicode properties (e.g., :Lu) into character sets by using + and - (e.g., <+:Lu - [AEIOU]>). However, they don't mention that you can also combine tokens/rules/regexes in with them, including ones you've defined yourself (e.g. <+my-token - :Lu>). This useful feature should be described.

(Additionally, that section of the docs could do a better job introducing the concept of character sets in general.)

For background, see Raku/problem-solving#97 and rakudo/rakudo#5430