Help: ENVIRONMENT_MODIFICATION: split modification ops

Refactoring the environment modification ops list out of the environment
modification help file.

Help/include/ENVIRONMENT_MODIFICATION_OPS.rst

@@ -0,0 +1,20 @@
+- ``reset``: Reset to the unmodified value, ignoring all modifications to
+  ``MYVAR`` prior to this entry. Note that this will reset the variable to
+  the value set by :prop_test:`ENVIRONMENT`, if it was set, and otherwise
+  to its state from the rest of the CTest execution.
+- ``set``: Replaces the current value of ``MYVAR`` with ``VALUE``.
+- ``unset``: Unsets the current value of ``MYVAR``.
+- ``string_append``: Appends singular ``VALUE`` to the current value of
+  ``MYVAR``.
+- ``string_prepend``: Prepends singular ``VALUE`` to the current value of
+  ``MYVAR``.
+- ``path_list_append``: Appends singular ``VALUE`` to the current value of
+  ``MYVAR`` using the host platform's path list separator (``;`` on Windows
+  and ``:`` elsewhere).
+- ``path_list_prepend``: Prepends singular ``VALUE`` to the current value of
+  ``MYVAR`` using the host platform's path list separator (``;`` on Windows
+  and ``:`` elsewhere).
+- ``cmake_list_append``: Appends singular ``VALUE`` to the current value of
+  ``MYVAR`` using ``;`` as the separator.
+- ``cmake_list_prepend``: Prepends singular ``VALUE`` to the current value of
+  ``MYVAR`` using ``;`` as the separator.

Help/prop_test/ENVIRONMENT_MODIFICATION.rst

@@ -13,26 +13,7 @@ where ``MYVAR`` is the case-sensitive name of an environment variable
 to be modified.  Entries are considered in the order specified in the
 property's value.  The ``OP`` may be one of:
 
-- ``reset``: Reset to the unmodified value, ignoring all modifications to
-  ``MYVAR`` prior to this entry. Note that this will reset the variable to
-  the value set by :prop_test:`ENVIRONMENT`, if it was set, and otherwise
-  to its state from the rest of the CTest execution.
-- ``set``: Replaces the current value of ``MYVAR`` with ``VALUE``.
-- ``unset``: Unsets the current value of ``MYVAR``.
-- ``string_append``: Appends singular ``VALUE`` to the current value of
-  ``MYVAR``.
-- ``string_prepend``: Prepends singular ``VALUE`` to the current value of
-  ``MYVAR``.
-- ``path_list_append``: Appends singular ``VALUE`` to the current value of
-  ``MYVAR`` using the host platform's path list separator (``;`` on Windows
-  and ``:`` elsewhere).
-- ``path_list_prepend``: Prepends singular ``VALUE`` to the current value of
-  ``MYVAR`` using the host platform's path list separator (``;`` on Windows
-  and ``:`` elsewhere).
-- ``cmake_list_append``: Appends singular ``VALUE`` to the current value of
-  ``MYVAR`` using ``;`` as the separator.
-- ``cmake_list_prepend``: Prepends singular ``VALUE`` to the current value of
-  ``MYVAR`` using ``;`` as the separator.
+ .. include:: ../include/ENVIRONMENT_MODIFICATION_OPS.rst
 
 Unrecognized ``OP`` values will result in the test failing before it is
 executed. This is so that future operations may be added without changing