docs: Add documentation for configuring clangd in C-only mode (#20255)

docs/src/languages/c.md

@@ -2,9 +2,14 @@
 
 C support is available natively in Zed.
 
-<!--
-TBD: Add instructions/configuration so clangd operates in C-only mode
--->
-
 - Tree Sitter: [tree-sitter/tree-sitter-c](https://github.com/tree-sitter/tree-sitter-c)
 - Language Server: [clangd/clangd](https://github.com/clangd/clangd)
+
+## Clangd: Force detect as C
+
+Clangd out of the box assumes mixed C++/C projects. If you have a C-only project you may wish to instruct clangd to all files as C using the `-xc` flag. To do this, create a `.clangd` file in the root of your project with the following:
+
+```yaml
+CompileFlags:
+  Add: [-xc]
+```