DOC: Make obvious that Qt.openUrlExternally, QDesktopServices::openUrl always returns true

The documentation for QDesktopServices::openUrl is somewhat vague on the return value.
The documentation for Qt.openUrlExternally is somewhat vague on the return value.

Both of these should be altered to indicate that it will always return true so long as the url conforms to rfc3986, or in some unforeseen case of catastrophic failure.

Original bug:
PRECONDITIONS:
A file with a filetype which is not associated to any app in the operating
system.

STEPS LEADING TO PROBLEM:
1. Call to Qt.openUrlExternally(filePath)

EXPECTED OUTCOME:
Returns false

ACTUAL OUTCOME:
Returns true
and following error message can be seen:
no default action for the given PARAMS