Convert builtin help to reStructuredText source files

Run the convert-help.bash script to convert documentation:

 ./convert-help.bash "/path/to/CMake-build/bin"

Then remove it.

Help/command/variable_requires.rst

@@ -0,0 +1,20 @@
+variable_requires
+-----------------
+
+Deprecated. Use the if() command instead.
+
+Assert satisfaction of an option's required variables.
+
+::
+
+  variable_requires(TEST_VARIABLE RESULT_VARIABLE
+                    REQUIRED_VARIABLE1
+                    REQUIRED_VARIABLE2 ...)
+
+The first argument (TEST_VARIABLE) is the name of the variable to be
+tested, if that variable is false nothing else is done.  If
+TEST_VARIABLE is true, then the next argument (RESULT_VARIABLE) is a
+variable that is set to true if all the required variables are set.
+The rest of the arguments are variables that must be true or not set
+to NOTFOUND to avoid an error.  If any are not true, an error is
+reported.