docs: add param'' ice description

[psprint]

Description

There is an undocumented ice param'' which allows to declare variables for the time of loading plugins.

Related Issue(s)

Motivation and Context

All ices should be documented.

Usage examples

zinit param'HOME -> /tmp' for zdharma-continuum/null

How Has This Been Tested?

It is a long existing ice used in e.g.: bin-gem-node annex.

Types of changes

• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Documentation change
• [ ] New feature (non-breaking change which adds functionality)

Checklist:

• [x] All new and existing tests passed.
• [ ] I have added tests to cover my changes.
• [x] I have updated the documentation accordingly.