Documentation: include supportedLngs in example

[dgmstuart]

Clarify that how the list of detected languages is used by i18next is dependent on the value of supportedLngs.

This wasn't clear to me from the existing documentation - the only reference I could find to that config option was in the big list of config options, and the description there doesn't refer to detection.

As discussed in issue https://github.com/i18next/i18next-browser-languageDetector/issues/283

Checklist (for documentation change)

• [x] only relevant documentation part is changed (make a diff before you submit the PR)
• [x] motivation/reason is provided
• [x] commit message and code follows the Developer's Certification of Origin

[adrai]

thx