[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

11

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

13

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

24

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

33

this example is insecure - include the whole prefix instead

37

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

42

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 that can change its behaviour in
+certain situations. This page aims to define those configuration knobs.
+
+Those command line arguments needs to be specified in an editor-specific way.
+You can find some editor specific instructions in `here <https://clang.llvm.org/extra/clangd/Installation.html#id3>`__.
+
+--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`).
+
+You can specify your driver as a list of globs or full paths, then clangd will
+execute drivers and fetch necessary include paths to compile your code.
+
+For example if you have your compilers at:
+ - `/path/to/my-custom/toolchain1/arm-none-eabi-gcc`,
+ - `/path/to/my-custom/toolchain2/arm-none-eabi-g++`,
+ - `/path/to/my-custom2/toolchain/arm-none-eabi-g++`,
+you can provide clangd with
+`--query-driver=/path/to/my-custom/**/arm-none-eabi*` to enable execution of
+any binary that has a name starting with `arm-none-eabi` and under
+`/path/to/my-custom/`. This won't allow execution of the last compiler.
+
+Full list of flags
+==================
+
+You can find out about the rest of the flags using `clangd --help`.

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