Add BuiltinHeaders to config docs

config.md

@@ -147,6 +147,29 @@ flag parsing (clang vs clang-cl), target inference (gcc-arm-noneabi) etc.
 If the option matches a glob mentioned in `--query-driver`, then it'll be
 invoked for extraction of include paths.
 
+### BuiltinHeaders
+{:.v21}
+
+Controls whether Clangd should include its own built-in headers (like
+stddef.h), or use the system header found from the query driver.
+
+Valid values are:
+- `Clangd`: Use builtin headers from `clangd`. This is the default.
+- `QueryDriver`: Use the headers extracted from the compiler via the
+  `--query-driver` command line argument. If a query driver is not supplied or
+  does not match the compiler, then the `Clangd` builtin headers will be the
+  fallback.
+
+```yaml
+CompileFlags:
+  BuiltinHeaders: QueryDriver
+```
+
+**Note**: if the driver is not clang, `BuiltinHeaders: QueryDriver` will result
+in the clang frontend (embedded in clangd) processing the builtin headers of
+another compiler, which could lead to unexpected results such as false positive
+diagnostics.
+
 ## Index
 
 Controls how clangd understands code outside the current file.

styles.css

@@ -170,7 +170,7 @@ a[href^="https://code.woboq.org/"] {
 /* Version marker ornaments */
 .v6::before, .v7::before, .v8::before, .v9::before, .v10::before, .v11::before, .v12::before,
 .v13::before, .v14::before, .v15::before, .v16::before, .v17::before, .v18::before, .v19::before,
-.v20::before {
+.v20::before, .v21::before {
   color: #008;
   border-radius: 3px;
   padding: 0.2em 0.6em;
@@ -197,6 +197,7 @@ a[href^="https://code.woboq.org/"] {
 .v18::before { content: "clangd-18"; }
 .v19::before { content: "clangd-19"; }
 .v20::before { content: "clangd-20"; }
+.v21::before { content: "clangd-21"; }
 #edit {
   text-decoration: none;
   position: absolute;