docs: describe that hint inputs and outputs are init-ed

[ivokub]

Description

It is not mentioned that inputs and outputs are already initialized and the user shouldn't leak the value. Clarified a bit.

Related: #998

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

How has this been tested?

Not tested

How has this been benchmarked?

Not benchmarked.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules