Documentation: typofixes

In addition to fixing trivial and obvious typos, be careful about
the following points:

 - Spell ASCII, URL and CRC in ALL CAPS;
 - Spell Linux as Capitalized;
 - Do not omit periods in "i.e." and "e.g.".

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>