Enhanced documentation to clearify the require option and added the option for symfony/symfony#9773
Formatted and minor improvements
Fixed explanation of field options
[#3338] Removing delete_empty details because this is first being committed to 2.3

Since the original PR had a mix of changes that should apply to 2.3+ and 2.5+, I've
merged the entire PR into 2.3 and this commit removes the 2.5+ specific documentation.
After merging into master (i.e. 2.5), I'll reverse this commit to put the delete_empty details back.