Help: Add missing docs for the ctest --progress option

These docs were missing from the changes that introduced the feature in
!2240.

Help/envvar/CTEST_PROGRESS_OUTPUT.rst

@@ -0,0 +1,14 @@
+CTEST_PROGRESS_OUTPUT
+---------------------
+
+Boolean environment variable that affects how :manual:`ctest <ctest(1)>`
+command output reports overall progress.  When set to 1, TRUE, ON or anything
+else that evaluates to boolean true, progress is reported by repeatedly
+updating the same line.  This greatly reduces the overall verbosity, but is
+only supported when output is sent directly to a terminal.  If the environment
+variable is not set or has a value that evaluates to false, output is reported
+normally with each test having its own start and end lines logged to the
+output.
+
+The ``--progress`` option to :manual:`ctest <ctest(1)>` overrides this
+environment variable if both are given.

Help/manual/cmake-env-variables.7.rst

@@ -53,5 +53,6 @@ Environment Variables for CTest
    /envvar/CTEST_INTERACTIVE_DEBUG_MODE
    /envvar/CTEST_OUTPUT_ON_FAILURE
    /envvar/CTEST_PARALLEL_LEVEL
+   /envvar/CTEST_PROGRESS_OUTPUT
    /envvar/CTEST_USE_LAUNCHERS_DEFAULT
    /envvar/DASHBOARD_TEST_FROM_CTEST

Help/manual/ctest.1.rst

@@ -35,6 +35,19 @@ Options
  which one should be tested.  Example configurations are "Debug" and
  "Release".
 
+``--progress``
+ Enable short progress output from tests.
+
+ When the output of ``ctest`` is being sent directly to a terminal, the
+ progress through the set of tests is reported by updating the same line
+ rather than printing start and end messages for each test on new lines.
+ This can significantly reduce the verbosity of the test output.
+ Test completion messages are still output on their own line for failed
+ tests and the final test summary will also still be logged.
+
+ This option can also be enabled by setting the environment variable
+ :envvar:`CTEST_PROGRESS_OUTPUT`.
+
 ``-V,--verbose``
  Enable verbose output from tests.