doc: Clarified 4.2 Pattern matching

Combined redundant paragraphs about the match expression and removed a
redundant example.
doc: Minor example formatting in 11 References
doc: Corrected example in 17.8 Deriving implementations for traits

Corrected example to to use Rand trait referenced in preceding
description and included an example using the Show trait to print ABC.
auto merge of #13907 : iliekturtles/rust/tutorial, r=alexcrichton

Some minor clarifications and fixes.