Switch to /// comments.

Per the discussion in WebAssembly/component-model#286, remove some `//`
comments which had been intended as non-documentation comments to `///`
documentation comments.

wit/terminal.wit

@@ -1,19 +1,21 @@
+/// Terminal input.
+///
+/// In the future, this may include functions for disabling echoing,
+/// disabling input buffering so that keyboard events are sent through
+/// immediately, querying supported features, and so on.
 interface terminal-input {
  /// The input side of a terminal.
  resource terminal-input;
-
- // In the future, this may include functions for disabling echoing,
- // disabling input buffering so that keyboard events are sent through
- // immediately, querying supported features, and so on.
 }
 
+/// Terminal output.
+///
+/// In the future, this may include functions for querying the terminal
+/// size, being notified of terminal size changes, querying supported
+/// features, and so on.
 interface terminal-output {
  /// The output side of a terminal.
  resource terminal-output;
-
- // In the future, this may include functions for querying the terminal
- // size, being notified of terminal size changes, querying supported
- // features, and so on.
 }
 
 /// An interface providing an optional `terminal-input` for stdin as a