hints docs: unify the descriptions of `hints.root()`

(Thanks to Láďa.)
Also try to stress that hints.set() only takes a pair and not more.
18 jobs for hints-doc in 8 minutes and 39 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #154887
amd64 linux docker
build:clang:linux:amd64

00:01:41

passed #154886
amd64 linux docker
build:linux:amd64

00:01:39

passed #154888
amd64 linux docker allowed to fail
srpm

00:00:58

 
  Test
passed #154895
amd64 linux docker
deckard:linux:amd64

00:06:35

passed #154902
privileged
distro:epel-7

00:02:24

passed #154901
privileged
distro:fedora-27

00:03:12

passed #154894
docker
doc

00:00:42

passed #154893
amd64 linux docker
installcheck:linux:amd64

00:01:36

passed #154896
amd64 linux docker
installcheck:valgrind:linux:amd64

00:02:00

passed #154890
docker
lint:c

00:02:46

failed #154891
docker allowed to fail
lint:clang-scan-build

00:01:13

passed #154889
docker
lint:lua

00:00:31

passed #154899
amd64 linux docker
respdiff:iter:tcp:linux:amd64

00:03:54

passed #154900
amd64 linux docker
respdiff:iter:tls:linux:amd64

00:03:58

passed #154898
amd64 linux docker
respdiff:iter:udp:linux:amd64

00:04:05

passed #154892
amd64 linux docker
test:linux:amd64

00:01:18

passed #154897
amd64 linux docker
test:linux:amd64:valgrind

00:01:17

 
  Coverage
passed #154903
amd64 linux docker
coverage

00:00:21

70.2%
 
Name Stage Failure
failed
lint:clang-scan-build Test
                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 warnings generated.
CCLD daemon/kresd
scan-build: 29 bugs found.
scan-build: Run 'scan-view /builds/knot/knot-resolver/scan-results/2018-07-30-135005-14-1' to examine bug reports.
Uploading artifacts...
scan-results: found 34 matching files
Uploading artifacts to coordinator... ok
id=154891 responseStatus=201 Created token=9rgxzUdK
ERROR: Job failed: exit code 1