Typo fixes to composite-data/vectors/adding-an-item.asciidoc and subsection contribution to sets

Typo fixes to composite-data/vectors/adding-an-item.asciidoc

Simple transpositions in spots. Updated last sentence to something I
consider more concise and clear.

 composite-data/sets/testing-set-membership/testing-set-membership.asciidoc

Add a small subsection mentioning keys being functionally equivalent to sets in membership tests.