Add variable explanations to manual

9 jobs for manual_add in 1 minute and 29 seconds (queued for 42 seconds)
Status Job ID Name Coverage
  Build
canceled #9534
build_binaries

passed #9535
build_doc

00:01:29

 
  Test And Pkg
skipped #9538
macOS
pkg_macOS
skipped #9536
pkg_source
skipped #9537
pkg_windows
skipped #9542
test_dynare++
skipped #9539
test_matlab
skipped #9541
allowed to fail manual
test_octave
skipped #9540
allowed to fail manual
test_old_matlab