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.
auto merge of #13907 : iliekturtles/rust/tutorial, r=brson

Some minor clarifications and fixes.