Update Search Documentation

doc/user/search/index.md

@@ -50,7 +50,7 @@ groups:
    available:
    - `=`: Is
    - `!=`: Is not ([Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/18059) in GitLab 12.7)
-1. Enter the text to filter the attribute by.
+1. Enter the text to [filter the attribute by](#filters-autocomplete).
 1. Repeat this process to filter by multiple attributes. Multiple attributes are joined by a logical
    `AND`.
 
@@ -110,6 +110,18 @@ the dropdown) **Approved-By** and select the user.
 
 ![Filter MRs by approved by](img/filter_approved_by_merge_requests_v13_0.png)
 
+## Filters autocomplete
+
+GitLab provides many filters across many pages (issues, merge requests, epics,
+and pipelines among others) which you can use to narrow down your search. When
+using the filter functionality, you can start typing characters to bring up
+relevant users or other attributes.
+
+For performance optimization, there is a requirement of a minimum of three
+characters to begin your search. For example, if you want to search for
+issues that have the assignee "Simone Presley", you'll need to type at
+least "Sim" before autocomplete gives any relevant results.
+
 ## Search history
 
 You can view recent searches by clicking on the little arrow-clock icon, which is to the left of the search input. Click the search entry to run that search again. This feature is available for issues and merge requests. Searches are stored locally in your browser.