Cut help descriptions at a sentence end, not at any dot

[zmofei]

first_sentence cut a parameter's spec description at its first ., so help stopped inside ids, numbers and "e.g.":

<tilesets>  Tileset ID(s) in the format `username
<lat>       Latitude in degrees (range -85
<quality>   Compression quality parameter appended to format (e

A sentence now ends at a full stop followed by white space, or at the end of the text, and e.g. / i.e. don't count. The unit test had pinned the old cut ("Tileset ID in the format \username"); it now pins the whole sentence, plus a decimal and an e.g.` case.

Verified: diffed -h for all 35 API commands before and after. Exactly 9 lines changed, all previously cut mid-word (tilesets, tilequery, static, styles download); nothing else moved. --schema is unaffected, since it never used this function.
Not checked: descriptions that use other abbreviations ending in "." mid-sentence; none in the bundled specs do today.

[zmofei]

Merged into #83 so all the help changes can be reviewed together.