`man bash` , search for "<<" see the "Here Strings" section.

You can also search https://www.google.com/search?q=bash and follow links to "Documentation"

    xargs echo <<EOL  
    > line 1
    > line 2  
    > EOL
    line 1 line 2

    $ cat <<EOL
    > line 1
    > line 2 
    > EOL
    line 1
    line 2

line-terminators get eaten by xargs