improved try documentation

- better `if` example
- Added chaining example to the try method description
- Documented the `respond_to?` check to the try method description
- Clearer wording to explain that argument error is raised on argument mismatch to responding method, rather than to non-responding method (which is handled without exception by `try`)
- `.any?` is more precise than `! .blank?`
 - Don't need to use `try` on `children` as (for regular associations) they will always be a collection or array that responds to `first` 
- Fix typos/grammar
@egilburg why was this changed?