Add some build reference doc for existing behavior (TriBITSPub#510, T…

…riBITSPub#511)

I noticed that some important TriBITS logic that was not documented in the
build reference guide.  I noticed this while working on TriBITSPub#510 and TriBITSPub#511 (but,
again, this is just documenting existing behavior).

tribits/doc/build_ref/TribitsBuildReferenceBody.rst

@@ -411,6 +411,8 @@ Both of these variables are automatically enabled when
 Enable a set of packages
 ++++++++++++++++++++++++
 
+.. _<Project>_ENABLE_ALL_OPTIONAL_PACKAGES:
+
 .. _<Project>_ENABLE_TESTS:
 
 To enable an SE package ``<TRIBITS_PACKAGE>`` (and optionally also its tests
@@ -448,6 +450,9 @@ statement in an input ```*.cmake`` options files.  However, setting
 ``-DXXX_ENABLE_YYY=TRUE`` and ``-DXXX_ENABLE_YYY=FALSE`` is allowed and will
 be interpreted correctly..
 
+NOTE: Setting ``<Project>_ENABLE_TESTS=ON`` also causes
+``<Project>_ENABLE_EXAMPLES=ON`` to be set by default as well.
+
 
 Enable or disable tests for specific packages
 +++++++++++++++++++++++++++++++++++++++++++++
@@ -502,6 +507,11 @@ of their tests turned on.  Tests will not be enabled in packages that do not
 depend on ``<TRIBITS_PACKAGE>`` in this case.  This speeds up and robustifies
 pre-push testing.
 
+NOTE: setting ``<Project>_ENABLE_ALL_FORWARD_DEP_PACKAGES=ON`` also
+automatically sets and overrides `<Project>_ENABLE_ALL_OPTIONAL_PACKAGES`_ to
+be ``ON`` as well.  (It makes no sense to want to enable forward dependent
+packages for testing purposes unless you are enabling all optional packages.)
+
 
 Enable all packages (and optionally all tests)
 ++++++++++++++++++++++++++++++++++++++++++++++