docs: Add user guide for *Advanced search* feature.

templates/zerver/help/advanced-search.md

@@ -0,0 +1,66 @@
+# Advanced search for messages
+
+In Zulip, you can find specific messages by using the search bar
+(highlighted in red in the previous image) at the top of your
+screen. This feature allows you to narrow your view to show specific
+messages using search constraints called **operators**.
+
+![Search bar](/static/images/help/search-bar.png)
+
+Operators are short phrases in the format `operator:operand` where
+`operand` represents the criteria that fits the specified
+operator. Operators allow you to instantly find messages that meet the
+specified criteria, such as messages that belong to a specific stream
+or topic.
+
+It is important to note that spaces in `operand` must be replaced with
+`+`. In addition, multiple search operators can be used in a single
+query by separating each operator with spaces in your search bar. Most
+importantly, you can search an individual stream for results from
+before you joined, but search results will only include messages from
+after you first joined Zulip if you search the entire realm.
+
+## Search operators
+
+Listed below are all Zulip search operators.
+
+* `stream:foobar` - This operator narrows the view to show only
+  messages in the stream `foobar`
+* `topic:foo+bar` - This operator narrows the view to show only
+  messages with the topic `foo bar`. It is important to note that the
+  complete topic name must be entered, or no results will be shown.
+* `pm-with:foo@bar.com` - This operator narrows the view to show only
+  private messages sent from the user with the email address
+  `foo@bar.com`.
+* `sender:foo@bar.com` - This operator narrows the view to show all
+  messages sent by the user with the email address `foo@bar.com`.
+* `sender:me` -This operator narrows the view to show all messages sent by you.
+* `near:xxxxx` - This operator narrows the view to show the message
+  with the ID `xxxxx` as well as a few messages sent before and after
+  the message.
+* `id:xxxxx` - This operator narrows the view to show only the message with the ID `xxxxx`.
+* `is:alerted` - This operator narrows the view to show messages with
+  alert words. You can add custom alert words in the **Settings** tab.
+* `is:mentioned` - This operator narrows the view to show messages that mention you.
+* `is:private` - This operator narrows the view to show all private messages that you've received.
+* `is:starred` - This operator narrows the view to show all messages that you've starred.
+* `has:link` - This operator narrows the view to show all messages that contain any links.
+* `has:image` - This operator narrows the view to show all messages that contain any images.
+* `has:attachment` - This operator narrows the view to show all messages that contain any attachments or uploads.
+* `keyword` - This operator narrows the view to show all messages containing the word or phrase `keyword`.
+* `"keyword"` - This operator narrows the view to show all messages containing the exact word or phrase `keyword`.
+* `-topic:foobar` - This operator narrows the view to exclude messages with the topic `foobar`
+
+## Keyboard shortcuts
+
+Listed below are some Zulip keyboard shortcuts that will aid you in conducting a search.
+
+* **Initiate a search** `/` - This shortcut moves the user's cursor to
+  the message search bar at the top of the window to allow them to
+  begin searching for messages belonging to a specific topic, stream,
+  view, etc. in the realm.
+* **Clear a search** `Esc` - This shortcut clears the search bar of
+  any search criteria that was previously entered. This action can
+  also be achieved by clicking the **x**
+  (![x](/static/images/help/x.png)) icon in the right side of the
+  search bar.

templates/zerver/help/index.md

@@ -71,6 +71,9 @@ as a “**realm**”.
 * Edit or delete a message
 * Change the topic of a message
 * Change the topic of a group of messages
+### Searching
+* [Search messages](/help/search-messages)
+* [Advanced search for messages](/help/advanced-search)
 
 ## People
 * See whether someone is here or away

templates/zerver/help/search-messages.md

@@ -93,3 +93,7 @@ messages, @-mentions, etc.
   e.g. clicking the stream `rabbits` will search for `stream:rabbits`.
 
 * The search executed by clicking on a link is visible in the search bar.
+
+### Learning more
+
+* See the article on [Advanced search for messages](/help/advanced-search).