Backport fix for bug CORE-4144 : Error "context already in use (BLR error)" when preparing a query with UNION 

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_1_Release@58393 65644016-39b1-43b1-bf79-96bc8fe82c15
Updated the docs.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_1_Release@58404 65644016-39b1-43b1-bf79-96bc8fe82c15