Improve test element documentation

Add missing ignore-blanks property, document "first" and "not_first"
qualifiers and fix some typos.
10 jobs for doc_match_qual_and_ignore_blank in 7 minutes and 41 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #6002536
gst-macos-10.15
meson macos

00:03:42

passed #6002533
meson-mingw-w64-build

00:04:37

manual #6002537
docker windows 1809 allowed to fail manual
meson msys2
passed #6002531
meson-shared-build

00:04:36

passed #6002532
meson-static-build

00:04:56

passed #6002534
docker windows 1809
meson vs2017 amd64

00:01:18

passed #6002535
docker windows 1809
meson vs2017 x86

00:01:19

passed #6002530
mingw-build

00:06:34

passed #6002528
shared-build

00:07:41

passed #6002529
static-build

00:06:23