Update function documentation for 4.0.0

Update function documentation for 4.0.0

Without this patch the function documentation is out of sync with the
functions contained in the standard library.  This commit updates the
functions.  I generated the list using this sequence:

    cd ~/src/puppet
    git checkout 3.1.1
    bundle exec puppet doc -r function > /tmp/puppet_functions.txt
    cd ~/src/stdlib
    bundle exec puppet doc -r function > /tmp/stdlib_functions.txt
    diff -U2 puppet_functions.txt stdlib_functions.txt | grep '^+' | perl -ple 's/^\+//' > functions.txt

I then replaced the README function documentation with the contents of
functions.txt which contains only the functions contained in stdlib.