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/write_file.rst

@@ -0,0 +1,20 @@
+write_file
+----------
+
+Deprecated. Use the file(WRITE ) command instead.
+
+::
+
+  write_file(filename "message to write"... [APPEND])
+
+The first argument is the file name, the rest of the arguments are
+messages to write.  If the argument APPEND is specified, then the
+message will be appended.
+
+NOTE 1: file(WRITE ...  and file(APPEND ...  do exactly the same as
+this one but add some more functionality.
+
+NOTE 2: When using write_file the produced file cannot be used as an
+input to CMake (CONFIGURE_FILE, source file ...) because it will lead
+to an infinite loop.  Use configure_file if you want to generate input
+files to CMake.