Help: Document what project() calls use CMAKE_PROJECT_INCLUDE and friends

Fixes: #19340

Help/variable/CMAKE_PROJECT_INCLUDE.rst

@@ -1,6 +1,9 @@
 CMAKE_PROJECT_INCLUDE
 ---------------------
 
-A CMake language file or module to be included by the :command:`project`
-command.  This is intended for injecting custom code into project
-builds without modifying their source.
+A CMake language file or module to be included as the last step of all
+:command:`project` command calls.  This is intended for injecting custom code
+into project builds without modifying their source.
+
+See also the :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE` and
+:variable:`CMAKE_PROJECT_INCLUDE_BEFORE` variables.

Help/variable/CMAKE_PROJECT_INCLUDE_BEFORE.rst

@@ -1,6 +1,9 @@
 CMAKE_PROJECT_INCLUDE_BEFORE
 ----------------------------
 
-A CMake language file or module to be included before processing the
-:command:`project` command. This is intended for injecting custom code into
-project builds without modifying their source.
+A CMake language file or module to be included as the first step of all
+:command:`project` command calls.  This is intended for injecting custom code
+into project builds without modifying their source.
+
+See also the :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE` and
+:variable:`CMAKE_PROJECT_INCLUDE` variables.

Help/variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE.rst

@@ -1,6 +1,10 @@
 CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE
 ------------------------------------
 
-A CMake language file or module to be included by the :command:`project`
-command.  This is intended for injecting custom code into project
-builds without modifying their source.
+A CMake language file or module to be included as the last step of any
+:command:`project` command calls that specify ``<PROJECT-NAME>`` as the project
+name.  This is intended for injecting custom code into project builds without
+modifying their source.
+
+See also the :variable:`CMAKE_PROJECT_INCLUDE` and
+:variable:`CMAKE_PROJECT_INCLUDE_BEFORE` variables.