docs: Update "Custom Keybindings for Extension-Based Agents section" to include a troubleshooting note for defining an agent name (#46144)

Iago Bermúdez created 

## Description
When trying to setup the OpenCode external agent the other day, I was
having trouble setting the keybinding using the docs. The issue was that
it said to use the same name as the one written in the UI. While this
works when the external agent is defined using the `settings.json` file,
it didn't work when setting up the keybinding after installing the
extension through the extension marketplace.

Since the solution I found was to define the agent name as 'opencode'
instead of 'OpenCode' (which is the name that shows up in the UI) I
thought it would be appropiate to indicate that sometimes you might need
to set it up using a different name.

Release Notes:

- N/A

Change summary

docs/src/ai/external-agents.md | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

Detailed changes

docs/src/ai/external-agents.md 🔗 

@@ -221,7 +221,7 @@ To assign a custom keybinding to start a new thread for agents that were added b
       {
         "agent": {
           "custom": {
-            "name": "My Agent", // The agent name as it appears in the UI (e.g., "OpenCode", "Auggie CLI", etc.)
+            "name": "My Agent", // The agent name as defined in the extension or in settings.json (e.g., "opencode", "Auggie CLI", etc.)
             "command": {
               "command": "my-agent", // The agent name in lowercase with no spaces
               "args": ["acp"]
@@ -234,6 +234,9 @@ To assign a custom keybinding to start a new thread for agents that were added b
 },
 ```
 
+> For most extensions, the `agent.custom.name` attribute matches the name of the agent that appears in the [Agent Panel](../ai/agent-panel.md) UI.
+> In some cases however, the name might need to be written differently (e.g. in lowercase).
+
 ## Debugging Agents
 
 When using external agents in Zed, you can access the debug view via with `dev: open acp logs` from the Command Palette. This lets you see the messages being sent and received between Zed and the agent.