Add warning/documentation about effects of operation outside the project during update process

Problem/Motivation

We need warning/documentation about effects of operation outside the project during update process

First example:
Composer post-update script would be fired in our stage environment. This might reference php files relative to project that aren't there, do destructive operations outside project, or might send notifications that should only happen in the active site.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes