Make it clear that rebuilding compile_commands.json is regularly needed.

Change-Id: I65c8b0d59b8d8d3dd70ef30da2586e06f18b3077
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226477
Auto-Submit: Fergal Daly <fergal@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774067}

docs/clangd.md

@@ -36,8 +36,9 @@ source file.
 tools/clang/scripts/generate_compdb.py -p out/Release > compile_commands.json
 ```
 
-Note: the compilation database is not re-generated automatically, you'd need to
-regenerate it manually when you have new files checked in.
+Note: the compilation database is not re-generated automatically. You need to
+regenerate it manually whenener build rules change, e.g when you have new files
+checked in or when you sync to head.
 
 If using Windows PowerShell, use the following command instead to set the
 output's encoding to UTF-8 (otherwise Clangd will hit "YAML:1:4: error: Got