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/export.rst

@@ -0,0 +1,44 @@
+export
+------
+
+Export targets from the build tree for use by outside projects.
+
+::
+
+  export(TARGETS [target1 [target2 [...]]] [NAMESPACE <namespace>]
+         [APPEND] FILE <filename> [EXPORT_LINK_INTERFACE_LIBRARIES])
+
+Create a file <filename> that may be included by outside projects to
+import targets from the current project's build tree.  This is useful
+during cross-compiling to build utility executables that can run on
+the host platform in one project and then import them into another
+project being compiled for the target platform.  If the NAMESPACE
+option is given the <namespace> string will be prepended to all target
+names written to the file.  If the APPEND option is given the
+generated code will be appended to the file instead of overwriting it.
+The EXPORT_LINK_INTERFACE_LIBRARIES keyword, if present, causes the
+contents of the properties matching
+(IMPORTED_)?LINK_INTERFACE_LIBRARIES(_<CONFIG>)? to be exported, when
+policy CMP0022 is NEW.  If a library target is included in the export
+but a target to which it links is not included the behavior is
+unspecified.
+
+The file created by this command is specific to the build tree and
+should never be installed.  See the install(EXPORT) command to export
+targets from an installation tree.
+
+The properties set on the generated IMPORTED targets will have the
+same values as the final values of the input TARGETS.
+
+::
+
+  export(PACKAGE <name>)
+
+Store the current build directory in the CMake user package registry
+for package <name>.  The find_package command may consider the
+directory while searching for package <name>.  This helps dependent
+projects find and use a package from the current project's build tree
+without help from the user.  Note that the entry in the package
+registry that this command creates works only in conjunction with a
+package configuration file (<name>Config.cmake) that works with the
+build tree.