remote modify [ssh]: Add doc about the parameter 'allow_agent' (#1815)

* remote modify [ssh]: Add doc about the parameter 'allow_agent'

This parameter was created to fix iterative/dvc#4186

* Update content/docs/command-reference/remote/modify.md

* minor: apply inline code

* Update content/docs/command-reference/remote/modify.md

Co-authored-by: Ruslan Kuprieiev <[email protected]>
Co-authored-by: Ivan Shcheklein <[email protected]>
Co-authored-by: Jorge Orpinel <[email protected]>

Author: 4 people

content/docs/command-reference/remote/modify.md

@@ -539,6 +539,14 @@ more information.
   $ dvc remote modify myremote gss_auth true
   ```
 
+- `allow_agent` - whether to use [SSH agents](https://www.ssh.com/ssh/agent)
+  (`true` by default). Setting this to `false` is useful when `ssh-agent` is
+  causing problems, such as a "No existing session" error:
+
+  ```dvc
+  $ dvc remote modify myremote allow_agent false
+  ```
+
 </details>
 
 <details>