Update Jira integration's regex field text

What does this MR do and why?

In response to customer feedback and the recent modification to the Jira integration regex field, we are revising the text below the regex field to mitigate any potential misunderstandings.

We're:

• adding a link to RE2 syntax, to support the field's values;
• updating documentation to reflect the newest regex.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout the branch
2. Visit any Jira's integration -> Settings/Integrations/Jira
3. Validate that new text is below the regex field, and the URL is pointing to https://github.com/google/re2/wiki/Syntax

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.