Explain potentially confusing string example.

Reported be @ElBaha
Fix span.
Note that Rust can be embedded.
Add a link to the wiki's package list and refer to git
Fix grammar error.
Add rustdoc documentation.
Fix tiny formatting error.
Clarify that strings aren't magical.
Clarify `as`, mention transmute.
Fix tiny formatting error.
Macro definition is feature gated.
Wildcard changes.
loop -> continue
Inherited mutabilty clarification

Also no longer reference mutable fields
Clarify arity
Clarify newtype
Function declaration takes pats, not exprs.