Move snapshot distribution explanation to separate man section.

Move snapshot distribution explanation to separate man section.

Since the new section contains a table, we have to tell groff to run
the tbl preprocessor to generate the html version of the man page.

We now pass -I image to the groff postprocessor so that
the png image file gets a predictable name. We'd love to
pass --image‐directory=build/ to place the png file in
the build directory, but this also creates html like <img
src="build/image1.png">, which does not work because index.html is
in build/. Change the cwd to the build directory to work around this.