docs: fix JSDoc documentation in external_api.js

[codewithabhay10]

Fixes #17087

Changes

• captureCameraPicture() — Fixed copy-paste bug: descriptionText and titleText param descriptions were incorrectly copied from cameraFacingMode
• executeCommand() — Added @param {...*} args for the rest parameter and documented 20+ available commands with their arguments. Added @example block
• setMediaEncryptionKey() — Fixed property name mismatch: keyInfo.encryptionKey → keyInfo.key to match actual code destructuring
• getAvailableDevices() — Added resolved value shape (audioInput, audioOutput, videoInput arrays) to @returns
• invite() — Documented invitee object properties (type, address, name) with @example

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.