[GH-2293]-Adding clarification of how data needs to be prefix

[axelgalicia]

Adding clarification of data prefix for sign method.

This clarification helps the users using the sign method that there is a need to prefix the data.

Fixes #2293

Type of change

• [ x] Bug fix

Checklist:

• [ x] I have selected the correct base branch.
• [ x] I have performed a self-review of my own code.
• [ x] I have commented my code, particularly in hard-to-understand areas.
• [ x] I have made corresponding changes to the documentation.
• [ x] My changes generate no warnings.
• [ x] I have updated or added types for all modules I've changed
• [ x] Any dependent changes have been merged and published in downstream modules.
• [ x] I ran npm run test in the root folder with success and extended the tests if necessary.
• [ x] I ran npm run build in the root folder and tested it in the browser and with node.
• [ x] I ran npm run dtslint in the root folder and tested that all my types are correct
• [ x] I have tested my code on the live network.

[axelgalicia]

Hello @nivida , I would like to be a formal contributor of the project. Do you know how can I do it?