doc: add trace category for fs sync methods

Add the trace category for file system synchronous methods to
documentation so the users can enable it when they want to look
into file system sync method trace data.

Refs: nodejs#19649

doc/api/tracing.md

@@ -23,6 +23,7 @@ The available categories are:
     measures and marks.
   * `node.perf.timerify` - Enables capture of only Performance API timerify
     measurements.
+* `node.fs.sync` - Enables capture of trace data for file system sync methods.
 * `v8` - The [V8] events are GC, compiling, and execution related.
 
 By default the `node`, `node.async_hooks`, and `v8` categories are enabled.