Document use cases and behavior of #[hdk_extern(infallible)] attribute

[c12i]

While working on holochain/holochain#4617, I noticed that the documentation could benefit from a section explaining the infallible attribute for #[hdk_extern].

This attribute is useful for zome functions that are guaranteed to not return errors or operations that can't fail. Adding documentation about when to use this attribute and how it affects the function's return type would help developers make better decisions about when to leverage this optimization and help distinguish it from the regular #[hdk_extern] usage where error handling is needed.

[pdaoust]

As I go I'm pointing out the infallible arg whenever I'm sharing about a callback that needs it. It's probably okay just introduced when/where it's needed, and it could point to the Rustdoc for more info. (Is there Rustdoc?) @c12i does that seem sufficient?