Doc: minor language and layout edits

Task-number: QTBUG-31801

Change-Id: I99043ba5d1a417e093ae2e4bc7d2967ec9d095fc
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Update QLineEdit::placeholderText property documentation

eeb31ad10a1c1b26233f383de6d1b2b217cde08f changed the behavior
and allowed the placeholder text to stay visible even if the lineedit
has focus.

Task-number: QTBUG-33237
Change-Id: I733331be1d3225eb28390b21eb9a7e4971bfdc31
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>