Document `ninja reconfigure` in the online documentation

After the build dir is created, I can change meson files or options and running ninja will reconfigure if required then build. My project checks that various options are compatible and prints a summary of the build configuration. I would like to be able to perform a reconfigure with out building so I can see the configuration summary before I decide if I want to build the project.

Even better would be the way cmake does it. I can change run "cmake .. -Dopt=var" in my build dir and it prints out my build summary. It seems to figure out how to only reconfigure what has changed instead of reconfiguring everything then prints out the summary. I assume all messages not inside a if statement are getting printed.