Correct QLineEdit::inputMask documentation.

Correct QLineEdit::inputMask documentation.

The documentation says:

"Unset the mask and return to normal QLineEdit operation by passing an
empty string ("") or just calling setInputMask() with no arguments."

However, the parameter for setInputMask is a const QString&, so calling
it with no arguments will result in a compile error. The documentation
should not advise calling setInputMask() without arguments.

Change-Id: Icd8e063dcd3d745bd4bf0c6851a38f63481957ae
Task-number: QTBUG-24803
Reviewed-by: Jerome Pasion <jerome.pasion@nokia.com>