Move the documentation of %{temp} to where other substitutions are documented

We should document everything in the same place, but for now let's be
consistent.

Author: ldionne

libcxx/docs/TestingLibcxx.rst

@@ -415,9 +415,6 @@ writing tests easier. See `libc++-specific Lit Directives`_ for more information
    * - Directive
      - Parameters
      - Usage
-   * - ``%{temp}``
-     - N/A
-     - This directive points to a temporary directory unique to the test.
    * - ``FILE_DEPENDENCIES``
      - ``// FILE_DEPENDENCIES: file, directory, /path/to/file, ...``
      - The paths given to the ``FILE_DEPENDENCIES`` directive can specify directories or specific files upon which a given test depend. For example, a test that requires some test

libcxx/utils/libcxx/test/format.py

@@ -261,6 +261,10 @@ class CxxStandardLibraryTest(lit.formats.FileBasedTest):
         %{run}
             Equivalent to `%{exec} %t.exe`. This is intended to be used
             in conjunction with the %{build} substitution.
+
+        %{temp}
+            This substitution expands to a non-existent temporary path unique to the test.
+            It is typically used to create a temporary directory.
     """
 
     def getTestsForPath(self, testSuite, pathInSuite, litConfig, localConfig):