New blog post: Props in v0.4

(cherry picked from commit 607de16d821ead28576c001d47e80394c12bde57)
Emphasis on single child.
(cherry picked from commit 858377946fe5d28773fe174071a1d3a922695a07)
Update syntax.md

Some inconsistencies pointed out in FB comments
(cherry picked from commit b6451be5823d199487f390997b0adc0f038dc79f)