[clang] Clarify documentation for -funique-source-file-names. (PR #135832)

[Peter Collingbourne via cfe-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/135832

None

>From 6d04789998482be11d15e8969c54df0375987a60 Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <peter at pcc.me.uk>
Date: Tue, 15 Apr 2025 11:27:48 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.6-beta.1
---
 clang/docs/UsersManual.rst | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index d4656a7e63c99..c75e409dde55e 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -2301,11 +2301,31 @@ are listed below.
 
    When enabled, allows the compiler to assume that each object file
    passed to the linker has been compiled using a unique source file
-   name. This is useful for reducing link times when doing ThinLTO
+   path. This is useful for reducing link times when doing ThinLTO
    in combination with whole-program devirtualization or CFI.
 
-   A misuse of this flag will generally result in a duplicate symbol
-   error at link time.
+   The full source path passed to the compiler must be unique. This
+   means that, for example, the following is a usage error:
+
+   .. code-block:: console
+
+     $ cd foo
+     $ clang -funique-source-file-names -c foo.c
+     $ cd ../bar
+     $ clang -funique-source-file-names -c foo.c
+     $ cd ..
+     $ clang foo/foo.o bar/foo.o
+    
+   but this is not:
+
+   .. code-block:: console
+
+     $ clang -funique-source-file-names -c foo/foo.c
+     $ clang -funique-source-file-names -c bar/foo.c
+     $ clang foo/foo.o bar/foo.o
+
+   A misuse of this flag will generally either be harmless or result
+   in a duplicate symbol error at link time.
 
 .. option:: -fforce-emit-vtables