docs: add deprecate to documentation (#1762)

docs/api.md

@@ -1191,6 +1191,7 @@ Valid `opt` keys include:
 - `default`: value, set a default value for the option, see [`default()`](#default)
 - `defaultDescription`: string, use this description for the default value in help content, see [`default()`](#default)
 - `demandOption`: boolean or string, demand the option be given, with optional error message, see [`demandOption()`](#demandOption)
+- `deprecate`/`deprecated`: boolean or string, mark option as deprecated, see [`deprecateOption()`](#deprecateOption)
 - `desc`/`describe`/`description`: string, the option description for help content, see [`describe()`](#describe)
 - `global`: boolean, indicate that this key should not be [reset](#reset) when a command is invoked, see [`global()`](#global)
 - `group`: string, when displaying usage instructions place the option under an alternative group heading, see [`group()`](#group)