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/utility_source.rst

@@ -0,0 +1,22 @@
+utility_source
+--------------
+
+Specify the source tree of a third-party utility.
+
+::
+
+  utility_source(cache_entry executable_name
+                 path_to_source [file1 file2 ...])
+
+When a third-party utility's source is included in the distribution,
+this command specifies its location and name.  The cache entry will
+not be set unless the path_to_source and all listed files exist.  It
+is assumed that the source tree of the utility will have been built
+before it is needed.
+
+When cross compiling CMake will print a warning if a utility_source()
+command is executed, because in many cases it is used to build an
+executable which is executed later on.  This doesn't work when cross
+compiling, since the executable can run only on their target platform.
+So in this case the cache entry has to be adjusted manually so it
+points to an executable which is runnable on the build host.