[clang] Document runtime config directory options (PR #66593)

[via cfe-commits]

https://github.com/xbjfk created https://github.com/llvm/llvm-project/pull/66593

In the clang user manual the build options `CLANG_CONFIG_FILE_USER_DIR` and `CLANG_CONFIG_FILE_SYSTEM_DIR` are documented, but the run time overrides `--config-user-dir` and `--config-system-dir` are not.

I have updated the manual to add these run time arguments.

>From c21b4281428e2a76b09c532ae1dc23101dd22c0e Mon Sep 17 00:00:00 2001
From: Reagan Bohan <xbjfk.github at gmail.com>
Date: Sun, 17 Sep 2023 21:19:33 +1200
Subject: [PATCH] Document runtime config directory options

---
 clang/docs/UsersManual.rst | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index 25bbd72c81f7a3b..e180f77793e80b7 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -944,10 +944,13 @@ treated as a file name and is searched for sequentially in the directories:
     - system directory,
     - the directory where Clang executable resides.
 
-Both user and system directories for configuration files are specified during
-clang build using CMake parameters, ``CLANG_CONFIG_FILE_USER_DIR`` and
-``CLANG_CONFIG_FILE_SYSTEM_DIR`` respectively. The first file found is used.
-It is an error if the required file cannot be found.
+Both user and system directories for configuration files can be specified
+either during build or during runtime. At build time, use
+``CLANG_CONFIG_FILE_USER_DIR`` and ``CLANG_CONFIG_FILE_SYSTEM_DIR``. At run
+time use the ``--config-user-dir=`` and ``--config-system-dir=`` command line
+options. Specifying config directories at runtime overrides the config
+directories set at build time The first file found is used. It is an error if
+the required file cannot be found.
 
 The default configuration files are searched for in the same directories
 following the rules described in the next paragraphs. Loading default