[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
Fixing old render syntax thanks to @oscherler
[#2781] Backporting by request
[#2832][#2775] Slight fix thanks to @WouterJ
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
Merge branch '2.2' into 2.3

Conflicts:
	reference/constraints/UniqueEntity.rst
Merge branch '2.3'
[#2797] Removing the 2.1 version notes on master (since 2.1 has been deprecated - see documentation release directions)