Added more explanation to docs/email.txt 'Preventing header injection' section.
Added more explanation to docs/email.txt 'Preventing header injection' section.

git-svn-id: 4f9f921b081c523744c7bf24d959a0db39629563@1923 bcc190cf-cafb-0310-a4f2-bffc1f526a37

t11e/django,bfirsh/django-old,rawwell/django,bfirsh/django-old,dcramer/django-compositepks,alex/django-old,mitsuhiko/django,disqus/django-old,django-nonrel/django-nonrel,jamespacileo/django-france,mitsuhiko/django,dcramer/django-compositepks,hugs/django,sam-tsai/django-old,hugs/django,alex/django-old,Smarsh/django,t11e/django,t11e/django,rawwell/django,skevy/django,Smarsh/django,dcramer/django-compositepks,sam-tsai/django-old,Instagram/django,jamespacileo/django-france,sam-tsai/django-old,django-nonrel/django-nonrel,disqus/django-old,Smarsh/django,mitsuhiko/django,Instagram/django,rawwell/django,skevy/django,jamespacileo/django-france,Smarsh/django,Instagram/django,disqus/django-old,alex/django-old,sam-tsai/django-old,hugs/django,django-nonrel/django-nonrel,bfirsh/django-old,skevy/django