Remove hint to security purpose

[jsmmo]

This is a doc PR related to: php/doc-en#34

[nikic]

To provide some context: Per our security policy, only issues that have a potential for remote exploitation are subject to the security policy. We consider the case where the attacker has full code execution privileges on the server to be a lost cause. Features like "open_basedir" and "disable_functions" are there to prevent mistakes, not to foil a determined "attacker". (Mistakes like: Accidentally deploying development tooling that performs shell calls.)

I started a discussion on removing open_basedir in the past, but the general consensus seems to be that this is still a helpful feature, even if it is understood that it is not a hard security guarantee.

So, this change is fine from my side, but I'm going to leave this PR open for a bit, in case there are more comments.