rename set_placeholder to set_placeholder_text and add more APIs

examples/popup_placeholder.rs

@@ -38,7 +38,7 @@ fn main() -> io::Result<()> {
     // set placeholder
 
     textarea.set_placeholder_style(Style::default());
-    textarea.set_placeholder("prompt message");
+    textarea.set_placeholder_text("prompt message");
     loop {
         term.draw(|f| {
             f.render_widget(textarea.widget(), area);

examples/ratatui_popup_placeholder.rs

@@ -36,7 +36,7 @@ fn main() -> io::Result<()> {
     };
     textarea.set_style(Style::default().fg(Color::Yellow));
     textarea.set_placeholder_style(Style::default());
-    textarea.set_placeholder("prompt message");
+    textarea.set_placeholder_text("prompt message");
     loop {
         term.draw(|f| {
             f.render_widget(textarea.widget(), area);

examples/single_line.rs

@@ -37,7 +37,7 @@ fn main() -> io::Result<()> {
 
     let mut textarea = TextArea::default();
     textarea.set_cursor_line_style(Style::default());
-    textarea.set_placeholder("enter a valid float, eg 1.56".to_string());
+    textarea.set_placeholder_text("Enter a valid float (e.g. 1.56)");
     let layout =
         Layout::default().constraints([Constraint::Length(3), Constraint::Min(1)].as_slice());
     let mut is_valid = validate(&mut textarea);