docs: add default activation behavior part

[Glatzel]

Description

pixi add some paths to PATH silently during activation, and cause some "valid issue". I add a part in docs to describe this and give the example and solution. Also, I add the link to global tools doc for users who meet this issue when using global tools like me.

I'm not sure if it covers all conditions, but my step to have a deeper understanding to pixi/conda mechanism. Hope it also help other users.

summary of the previous discuss #5571 :

• The issue is possible. The heavy pixi global tools users, like me :), are tend to meet this issue.
• The issue is a very edge case, but easy to reproduce.
• It is valid to pixi/conda, but it might not be the real intension of user.
• The issue will never be solved, so add something in docs would help if someone meet the same problem.

How Has This Been Tested?

I have run the example in the doc on my computer, and check the rattler and pixi source code, trampoline config file.

Checklist:

• I have performed a self-review of my own code
• I have added sufficient tests to cover my changes.

[baszalmstra]

Should this still be in draft?