[Cookbook][Session] Fixed headline in proxy_examples
added documentation for errorPath option of UniqueEntity constraint
added new line to errorPath documentation of UniqueEntity constraint
Some corrections of errorPath documentation for UniqueEntity constraint
Merge pull request #2770 from iamdto/fix-proxy_examples_headlines

[Cookbook][Session] Fixed headline in proxy_examples
[#2781] Backporting by request
Merge pull request #2797 from naitsirch/error_path

Added documentation for errorPath option of UniqueEntity constraint
Fixing bad link
[#2797] Minor tweaks to new UniqueEntity.errorPath docs
Merge branch '2.1' into 2.2