Document the new --use-local-ssh-agent flag for tsh

The flag is used to bypass the local SSH agent even when it's running.
Specifically, this helps with agents that don't support certs.

The flag was added in #3721

Updates #3169

docs/4.3/admin-guide.md

@@ -1266,11 +1266,14 @@ $ tsh login --proxy=proxy.example.com joe
 $ ssh-add -L
 ```
 
-!!! warning "GNOME Keyring SSH Agent"
-
-    It is well-known that Gnome Keyring SSH
-    agent, used by many popular Linux desktops like Ubuntu, does not support SSH
-    certificates. We recommend using the `ssh-agent` command from `openssh-client` package.
+!!! warning "GNOME Keyring SSH Agent and GPG Agent"
+
+    It is well-known that Gnome Keyring SSH agent, used by many popular Linux
+    desktops like Ubuntu, and gpg-agent from GnuPG do not support SSH
+    certificates. We recommend using the `ssh-agent` from OpenSSH.
+    Alternatively, you can disable SSH agent integration entirely using
+    `--no-use-local-ssh-agent` flag or `TELEPORT_USE_LOCAL_SSH_AGENT=false`
+    environment variable with `tsh`.
 
 ### OpenSSH Rate Limiting
 

docs/4.3/user-manual.md

@@ -149,6 +149,10 @@ $ ssh-add -L
 SSH agent can be used to feed the certificate to other SSH clients, for example
 to OpenSSH `ssh`.
 
+If you wish to disable SSH agent integration, pass `--no-use-local-ssh-agent`
+to `tsh`. You can also set the `TELEPORT_USE_LOCAL_SSH_AGENT` environment
+variable to `false` in your shell profile to make this permanent.
+
 ### Identity Files
 
 [`tsh login`](cli-docs.md#tsh-login) can also save the user certificate into a