Copyedits and punctuation (opt 2a).

Followup to comment on [PR 1842](https://github.com/jashkenas/underscore/pull/1842):

1801: Add missing full stop (period). 
1831 & 1835–6: Hyphenate compound modifiers. 
1850: Delete extra space character.
1860–1: Delete extra space character. No change to wording otherwise.
Merge pull request #1843 from janaspage/patch-3

Copyedits and punctuation (opt 2a).