Improve docs around Val conversion in collections

[mootz12]

What

Improve documentation for Vec and Map to clarify that element types in host-backed collections are validated lazily — only when accessed — not when the collection is constructed or received as a contract argument.

Why

Val conversion / type guarantees were documented for both the Vec and Map structs, but was missing on some of the utility functions where validation was unclear.

[Copilot]

Pull request overview

This PR improves Soroban SDK collection documentation to clarify the semantics of iter() vs try_iter() for Vec and Map, especially around lazy Val→typed conversion and when panics can occur.

Changes:

• Added/expanded rustdoc for Vec::{iter, try_iter} describing conversion behavior and panic semantics.
• Updated Map rustdoc and added docs for Map::{keys, values, iter, try_iter}.
• Expanded iter module-level docs to explain the “panicking” vs “fallible” iterator wrappers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
soroban-sdk/src/vec.rs	Adds docs for iter()/try_iter() behavior and panic guidance.
soroban-sdk/src/map.rs	Updates Map-level docs and documents keys()/values() and iterator variants.
soroban-sdk/src/iter.rs	Adds module docs explaining iterator design and error handling.

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