Backport the section about syntax of literals to doc/user_guide.txt
Backport the section about syntax of literals to doc/user_guide.txt

The section comes from the gitbook instance.

arpunk/lfe,bsmr-erlang/lfe,yurrriq/lfe,oubiwann/lfe,jjl/lfe,jjl/lfe,yurrriq/lfe,oubiwann/lfe,bsmr-erlang/lfe,arpunk/lfe