config docs: document valid fsmonitor values

I was wondering how to disable the watchman in a repo if it's enabled in
the user config.

cli/src/config-schema.json

@@ -155,6 +155,16 @@
                 }
             }
         },
+        "core": {
+            "type": "object",
+            "properties": {
+                "fsmonitor": {
+                    "type": "string",
+                    "enum": ["none", "watchman"],
+                    "description": "Whether to use an external filesystem monitor, useful for large repos"
+                }
+            }
+        },
         "colors": {
             "type": "object",
             "description": "Mapping from jj formatter labels to colors",

docs/config.md

@@ -718,6 +718,9 @@ In large repositories, it may be beneficial to use a "filesystem monitor" to
 track changes to the working copy. This allows `jj` to take working copy
 snapshots without having to rescan the entire working copy.
 
+This is governed by the `core.fsmonitor` option. Currently, the valid values are
+`"none"` or `"watchman"`.
+
 ### Watchman
 
 To configure the Watchman filesystem monitor, set