Add link to FindPython3.cmake and fix typo for unsetting Python3_EXEC…

…UTABLE (#610)

This typo did not get discovered on the PR review that added this.

tribits/doc/build_ref/TribitsBuildReferenceBody.rst

@@ -2238,11 +2238,11 @@ To set which Python interpreter is used, configure with::
   -D Python3_EXECUTABLE=<python-path>
 
 Otherwise, Python will be found automatically by default using
-``find_python(Python3)`` internally.
+``find_python(Python3)`` internally (see `FindPython3.cmake`_).
 
-To disable the finding and usage of Python, configure with::
+To disable the finding and usage of Python, configure with (empty)::
 
-  -D Python3_EXECUTABLE=<python-path>
+  -D Python3_EXECUTABLE=
 
 
 Outputting package dependency information
@@ -4505,4 +4505,7 @@ dashboard``.
 
 .. _xSDK Community Package Policies: https://doi.org/10.6084/m9.figshare.4495136
 
+.. _FindPython3.cmake: https://cmake.org/cmake/help/latest/module/FindPython3.html
+
+
 ..  LocalWords:  templated instantiation Makefiles CMake