[clang] [Clang][Docs][NFC] Correct documentation for the CPATH environment variable (PR #129113)

[Erich Keane via cfe-commits]

================
@@ -733,16 +733,17 @@ ENVIRONMENT
 
 .. envvar:: CPATH
 
-  If this environment variable is present, it is treated as a delimited list of
-  paths to be added to the default system include path list. The delimiter is
-  the platform dependent delimiter, as used in the PATH environment variable.
-
-  Empty components in the environment variable are ignored.
+  This environment variable specifies additional header file search paths which
+  behave as if they were specified with the :option:`-I\<directory\>` option at
+  the end of the driver command line. Paths are delimited by the platform
----------------
erichkeane wrote:

I don't mind a quick 'high level' view of what that means, even if it means copying slightly from the -I option.  As far as the GCC docs, those aren't particularly good either, so we can aim for better than that.

https://github.com/llvm/llvm-project/pull/129113