Skip_when_empty: Unobvious serialization behaviour | update docs

[Khairullin]

Q	A
Bug report?	no
Feature request?	no
Doc updated?	yes
Unobvious behaviour?	yes

Steps required to reproduce the problem

1. Create model class at least with one array field, initialize it with [], just when declaring field.
2. Write yaml config for model class, with xml_list option for array field
3. Post some test data and try to serialize it into xml format.
4. xml-tag for empty array abscent, it was skipped by default.
5. Try to serialize to json-format. json-field with empty array is present

Expected Result

Same behaviour for json and xml serialization for same yaml config. "skip_when_empty" option mentioned in docs as "allowed inside xml_list"

Actual Result

Serialization behaviour differs. No info in docs about "skip_when_empty" inside "xml_list", even in examples.

[goetas]

you are right "skip_when_empty" works well only for json. if you are interested to make it work for php, it would be great