Update our developer docs to state that Gitaly calls should not take place within a PG transaction

Problem to solve

In a recent Slack conversation in #g_gitaly it was stated:

@DylanGriffith:

It seems risky to keep a postgres transaction open while writing to external systems as it can cause performance problems due locks or possibly deadlocks.

@jacobvosmaer-gitlab:

you definitely do not want to make gitaly calls during a PG transaction. especially not gitaly writes, which run git hooks, which may run custom hooks, which may take forever

We should document this in https://docs.gitlab.com/ee/development/gitaly.html.