[clangd] Add docs for configuration knobs in clangd

[kadircet]

Summary

This is a first step in documenting different configuration knobs we
have in clangd.

[sammccall]

clang-tools-extra/docs/clangd/Configuration.rst

10

LSP capabilities generally aren't user-servicable and probably shouldn't be covered here.

12

Maybe mention that clangd command-line options should be specified in an editor-specific way, and point back to the getting started page for details

23

Skip the rationale here IMO - just "Specify your driver as a full path or list of globs,..."

32

this example is insecure - include the whole prefix instead

36

(I think we can drop this sentence, it seems obvious)

41

I'd point them at clangd --help rather than the code.

[kadircet]

• Address comments

clang-tools-extra/docs/clangd/Configuration.rst

+========
+Configuration
+========
+
+.. contents::
+
+.. role:: raw-html(raw)
+   :format: html
+
+Clangd has a bunch of command-line options and LSP capabilities that can change
+its behaviour in certain situations. This page aims to define those
+configuration knobs.
+
+--query-driver
+==============
+
+Clangd makes use of clang behind the scenes, so it might fail to detect your
+standard library or built-in headers if your project is making use of a custom
+toolchain. That is quite common in hardware-related projects, especially for the
+ones making use of gcc (e.g. ARM's `arm-none-eabi-gcc`).
+
+To get over this clangd can query your toolchain driver to extract implicit
+include paths that are unknown to clang. Since this requires executing a binary,
+we require users of this feature to whitelist their driver using the
+`--query-driver` flag. It accepts a comma-separated list of globs, which will
+match against the compiler driver paths specified in your
+`compile_commands.json`.
+
+For example if you have your compilers at
+`/path/to/my-custom-toolchain/arm-none-eabi-gcc` and
+`/path/to/my-custom-toolchain/arm-none-eabi-g++`; you
+can provide clangd with `--query-driver=**/my-custom-toolchain/arm-none-eabi*`
+to enable execution of any binary that has a name starting with `arm-none-eabi`
+and under a directory called `my-custom-toolchain`.
+
+By default this flag is empty, which disallows execution of any driver.
+
+Full list of flags
+==================
+
+You can find rest of the flags in `source code <https://github.com/llvm/llvm-project/blob/master/clang-tools-extra/clangd/tool/ClangdMain.cpp>`__.

clang-tools-extra/docs/clangd/index.rst

 ======
 clangd
 ======
 
 .. toctree::
    :maxdepth: 1
 
    Installation
    Features
+   Configuration
 
 What is clangd?
 ===============
 
 clangd understands your C++ code and adds smart features to your editor: code
 completion, compile errors, go-to-definition and more.
 
 clangd is a language server that implements the `Language Server Protocol