plugin: include a warning about race detector compatability in docs

As described in #24245, the race detector isn't able to find some race conditions in plugins.
This commit adds a warning about this to the plugins documentation.

Updates #24245

Change-Id: I1a5ab9b5e22b6a254efc639c3c03f75b79214d49
GitHub-Last-Rev: a5b4ff4
GitHub-Pull-Request: #68792
Reviewed-on: https://go-review.googlesource.com/c/go/+/604275
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: ExtraE113

src/plugin/plugin.go

@@ -29,6 +29,10 @@
 //     macOS, making them unsuitable for applications intended to be
 //     portable.
 //
+//   - Plugins are poorly supported by the Go race detector. Even simple
+//     race conditions may not be automatically detected. See
+//     https://go.dev/issue/24245 for more information.
+//
 //   - Applications that use plugins may require careful configuration
 //     to ensure that the various parts of the program be made available
 //     in the correct location in the file system (or container image).