[javadoc] fix some javadoc errors in javax/swing/plaf/synth

javadoc errors should be fixed in the following files:

SynthButtonUI.java
SynthCheckBoxMenuItemUI.java
SynthCheckBoxUI.java
SynthColorChooserUI.java
SynthComboBoxUI.java
SynthDesktopIconUI.java
SynthDesktopPaneUI.java
SynthEditorPaneUI.java
SynthFormattedTextFieldUI.java
SynthInternalFrameUI.java
SynthLabelUI.java
SynthListUI.java
SynthMenuBarUI.java
SynthMenuItemUI.java
SynthMenuUI.java
SynthOptionPaneUI.java
SynthPanelUI.java
SynthPasswordFieldUI.java
SynthPopupMenuUI.java
SynthProgressBarUI.java
SynthRadioButtonMenuItemUI.java
SynthRadioButtonUI.java
SynthRootPaneUI.java
SynthScrollBarUI.java
SynthScrollPaneUI.java
SynthSeparatorUI.java
SynthSliderUI.java
SynthSpinnerUI.java
SynthSplitPaneUI.java
SynthTabbedPaneUI.java
SynthTableHeaderUI.java
SynthTableUI.java
SynthTextAreaUI.java
SynthTextFieldUI.java
SynthTextPaneUI.java
SynthToggleButtonUI.java
SynthToolBarUI.java
SynthToolTipUI.java
SynthTreeUI.java
SynthTreeUI.java
SynthViewportUI.java
SynthMenuLayout.java