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/elseif.rst

@@ -0,0 +1,10 @@
+elseif
+------
+
+Starts the elseif portion of an if block.
+
+::
+
+  elseif(expression)
+
+See the if command.