Improvement docs for Cypher Procedures on 5.11

[conker84]

See the commit message of the PR:

GlobalProcedures produces immutable snapshots #21568

.. of the available procedures. These are updated on every modification to the ProcedureRegistry (~56 of those during a normal startup), and are thus fairly costly. Since this adds time mainly to the boot-up process, it is unlikely to cause any substantial issues.

This commit will introduce a "breaking" change for apoc-extended. If the user registers a procedure with apoc.custom.declareProcedure, and attempts to use it in the same transaction, where it would previously work(-ish) it will now fail since the procedure snapshot will not be updated for an ongoing transaction.

We need to update the docs accordingly.