feat(gatsby): Add note to DEV_SSR flag about custom webpack config (#31063)

If someone modifies their config for the `build-html` stage and haven't for `develop-html`, they'll probably also need to update it to modify `develop-html` stage as well #28138 (comment)

Author: KyleAMathews

packages/gatsby/src/utils/flags.ts

@@ -92,7 +92,7 @@ const activeFlags: Array<IFlag> = [
     command: `develop`,
     telemetryId: `DevSsr`,
     experimental: false,
-    description: `Server Side Render (SSR) pages on full reloads during develop. Helps you detect SSR bugs and fix them without needing to do full builds.`,
+    description: `Server Side Render (SSR) pages on full reloads during develop. Helps you detect SSR bugs and fix them without needing to do full builds. See umbrella issue for how to update custom webpack config.`,
     umbrellaIssue: `https://gatsby.dev/dev-ssr-feedback`,
     testFitness: (): fitnessEnum => {
       if (sampleSiteForExperiment(`DEV_SSR`, 20)) {