Help: Document special "CTEST_FULL_OUTPUT" test output string

Help/variable/CTEST_CUSTOM_MAXIMUM_FAILED_TEST_OUTPUT_SIZE.rst

@@ -5,4 +5,7 @@ When saving a failing test's output, this is the maximum size, in bytes, that
 will be collected by the :command:`ctest_test` command. Defaults to 307200
 (300 KiB).
 
+If a test's output contains the literal string "CTEST_FULL_OUTPUT",
+the output will not be truncated and may exceed the maximum size.
+
 .. include:: CTEST_CUSTOM_XXX.txt

Help/variable/CTEST_CUSTOM_MAXIMUM_PASSED_TEST_OUTPUT_SIZE.rst

@@ -5,4 +5,7 @@ When saving a passing test's output, this is the maximum size, in bytes, that
 will be collected by the :command:`ctest_test` command. Defaults to 1024
 (1 KiB).
 
+If a test's output contains the literal string "CTEST_FULL_OUTPUT",
+the output will not be truncated and may exceed the maximum size.
+
 .. include:: CTEST_CUSTOM_XXX.txt