docs: add once option on watcher (#2698)

* Update watchers.md

add once option description on watcher

* docs: column spacing

src/guide/essentials/watchers.md

@@ -272,6 +272,42 @@ watch(
 
 </div>
 
+
+## Once Watchers <sup class="vt-badge" data-text="3.4+" /> {#once-watchers}
+
+Watcher's callback will execute whenever the watched source changes. If you want the callback to trigger only once when the source changes, use the `once: true` option.
+
+<div class="options-api">
+
+```js
+export default {
+ watch: {
+ source: {
+ handler(newValue, oldValue) {
+ // when `source` changes, triggers only once
+ },
+ once: true
+ }
+ }
+}
+```
+
+</div>
+
+<div class="composition-api">
+
+```js
+watch(
+ source,
+ (newValue, oldValue) => {
+ // when `source` changes, triggers only once
+ },
+ { once: true }
+)
+```
+
+</div>
+
 <div class="composition-api">
 
 ## `watchEffect()` \*\* {#watcheffect}