[spec/function] Fix Safe Interface definition

[ntrel]

1. Mention safe context and globals for preconditions.
   Part of Bugzilla 24098 - Safe variable can be initialized from @system static constructor.

2. @trusted functions can create unsafe values/aliasing so long as they are not accessible from @safe code. E.g. setting a @system field.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	24098	normal	Safe variable can be initialized from `@system` static constructor.