[Clang][Docs][NFC] Correct documentation for the CPATH environment variable.

Clang's current documentation for the `CPATH` environment variable states that
paths it specifies are added as system header search paths. The actual behavior
is that such paths are treated as though they were passed via `-I` options at
the end of the driver command line and are thus added as non-system (user)
header search paths. Clang's behavior is consistent with gcc as documented at
https://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html#index-CPATH.
This change aligns Clang's documentation with the behavior actually observed.
Additional editorial changes are included to clarify that the related
`C_INCLUDE_PATH`, `CPLUS_INCLUDE_PATH`, `OBJC_INCLUDE_PATH`, and
`OBJCPLUS_INCLUDE_PATH` environment variables specify additional paths that are
treated as system header search paths (in contrast to `CPATH`).

Author: tahonermann

clang/docs/CommandGuide/clang.rst

@@ -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
+  dependent delimiter as used in the ``PATH`` environment variable. Empty
+  components in the environment variable are ignored.
 
 .. envvar:: C_INCLUDE_PATH, OBJC_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJCPLUS_INCLUDE_PATH
 
-  These environment variables specify additional paths, as for :envvar:`CPATH`, which are
-  only used when processing the appropriate language.
+  These environment variables specify additional system header file search
+  paths to be used when processing the corresponding language. Search paths are
+  delimited as for the :envvar:`CPATH` environment variable.
 
 .. envvar:: MACOSX_DEPLOYMENT_TARGET