Add variable explanations to manual

9 jobs for manual_add in 10 minutes and 6 seconds (queued for 13 minutes and 33 seconds)
Status Job ID Name Coverage
  Build
canceled #9599
build_binaries

00:10:06

passed #9600
build_doc

00:01:56

 
  Test And Pkg
canceled #9603
macOS
pkg_macOS

canceled #9601
pkg_source

canceled #9602
pkg_windows

canceled #9607
test_dynare++

canceled #9604
test_matlab

canceled #9606
allowed to fail manual
test_octave

canceled #9605
allowed to fail manual
test_old_matlab