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

[Tom Honermann via cfe-commits]

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@@ -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
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tahonermann wrote:

Hmm, I don't have any particularly good ideas that wouldn't duplicate what is (or should be) in the referenced description of the `-I` option. What I have now is pretty similar to what [gcc documents](https://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html#index-CPATH).

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