webui: String input now shows link to ALT docs

Some people might find the string input confusing. So we added a link to
ALT's documentation of `string::Parse` language.

The link is revealed only when string input is selected in the form.

Closes #45
9 jobs from staging in 9 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #191867
prepare:activemqcpp

00:01:03

 
  Build
passed #191869
build:webui

00:03:21

passed #191868
build:worker

00:00:49

 
  Test
passed #191871
test:webui

00:01:03

passed #191870
test:worker

00:00:22

 
  Deploy
passed #191873
deploy:client:staging

00:04:14

passed #191872
deploy:server:staging

00:02:11

passed #191874
deploy:worker:staging

00:01:59

 
  Notify
passed #191875
notify:altvm

00:00:12