Add `regexp/letter-case`/`unicorn/escape-case` conflict resolution to the doc

[yvele]

Information:

• ESLint version: 8.57.0
• eslint-plugin-regexp version: 2.6.0

Description

With this rule elabled:

"regexp/letter-case" : "error"

The following code:

const REG = /\u036F/;

is reported as error:

error '\u036F' is not in lowercase regexp/letter-case

but should be automatically fixed to:

- const REG = /\u036F/;
+ const REG = /\u036f/;

Same problem with the u flag /\u036F/u

Note that the rule doc specifies that:

🔧 This rule is automatically fixable by the --fix CLI option.