Doc: minor language edits

Concerning vocab., punctuation, syntax and/or Writing Guidelines.
Corrected whitespace issues.
Replaced "if" with "whether" in Slider.qml

Task-number: QTBUG-30318

Change-Id: I0e0e8d2b807f0a6534deb2caa6dbee7eb206f43b
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>