Add missing documentation for include dirs related variables. (#12611)

Document CMAKE_INCLUDE_DIRECTORIES_BEFORE and
CMAKE_INCLUDE_DIRECTORIES_PROJECT_BEFORE.

Help/command/include_directories.rst

@@ -19,7 +19,7 @@ used by the generators.
 
 By default the directories are appended onto the current list of
 directories.  This default behavior can be changed by setting
-CMAKE_INCLUDE_DIRECTORIES_BEFORE to ON.  By using AFTER or BEFORE
+:variable:`CMAKE_INCLUDE_DIRECTORIES_BEFORE` to ON.  By using AFTER or BEFORE
 explicitly, you can select between appending and prepending,
 independent of the default.
 

Help/manual/cmake-variables.7.rst

@@ -97,6 +97,8 @@ Variables that Change Behavior
    /variable/CMAKE_FIND_NO_INSTALL_PREFIX
    /variable/CMAKE_IGNORE_PATH
    /variable/CMAKE_INCLUDE_PATH
+   /variable/CMAKE_INCLUDE_DIRECTORIES_BEFORE
+   /variable/CMAKE_INCLUDE_DIRECTORIES_PROJECT_BEFORE
    /variable/CMAKE_INSTALL_DEFAULT_COMPONENT_NAME
    /variable/CMAKE_INSTALL_PREFIX
    /variable/CMAKE_LIBRARY_PATH

Help/variable/CMAKE_INCLUDE_DIRECTORIES_BEFORE.rst

@@ -0,0 +1,8 @@
+CMAKE_INCLUDE_DIRECTORIES_BEFORE
+--------------------------------
+
+Whether to append or prepend directories by default in :command:`include_directories`.
+
+This variable affects the default behavior of the :command:`include_directories`
+command. Setting this variable to 'ON' is equivalent to using the BEFORE option
+in all uses of that command.

Help/variable/CMAKE_INCLUDE_DIRECTORIES_PROJECT_BEFORE.rst

@@ -0,0 +1,8 @@
+CMAKE_INCLUDE_DIRECTORIES_PROJECT_BEFORE
+----------------------------------------
+
+Whether to force prepending of project include directories.
+
+This variable affects the order of include directories generated in compiler
+command lines.  If set to 'ON', it causes the :variable:`CMAKE_SOURCE_DIR` and
+the :variable:`CMAKE_BINARY_DIR` to appear first.