docs/manual: fix minor spelling/grammar mistakes

This commit fixes a few minor spelling or grammar mistakes since the recent
additions to the manual (commits 0b100de2 to
7cbb4766).

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>