js: documentation

• Fixed errors:
• src/Message.js post(): Set chunksize to defined default value instead of hardcoded
• src/Keys.js: added getHasSecret() to refreshKey operation.

• Reviewed and updated the documentation

• non-documentation changes which do not affect functionality:
• src/Errors: disabled a console.warn that is only useful for debugging
• helpers.js: renamed "string" to "value" in isFingerprint and isLongId to avoid confusion
• src/Keyring: prepare_sync, search are both explicitly set to false by default