Document Numbro literal and percentage format

[eradman]

Numbro.js formats percentages as value * 100, but any litteral prefix or suffix may be added with braces.

This is a partial solution to https://github.com/getredash/redash/issues/6586

Note: this change to documentation is not backward compatible, so this PR may have to wait for the next Redash release.

[eradman]

Cc @willvictor @masayuki038

[eradman]

This is no longer relevant, we reverted the change to numbro