use SAGE's help() function to look up doc.

Ideally, would use SAGE's ?, but I do not know how to call it from
the command line instead of SAGE's REPL

ftplugin/sage.vim

@@ -10,4 +10,20 @@ compiler sage
 
 let b:undo_ftplugin =  'setlocal tabstop< shiftwidth< expandtab<'
 let b:undo_ftplugin .= '| setlocal makeprg< errorformat<'
+
+if executable('sage')
+  if !has('gui_running')
+    command! -buffer -nargs=1 SageHelp
+          \ silent exe '!' . 'sage -c ''help(<args>)''' |
+          \ redraw!
+  elseif has('terminal')
+    command! -buffer -nargs=1 SageHelp
+          \ silent exe 'term ' . 'sage -c ''help(<args>)'''
+  else
+    command! -buffer -nargs=1 SageHelp echo system('sage -c ''<args>'' 2>/dev/null')
+  endif
+  setlocal keywordprg=:SageHelp
+  let b:undo_ftplugin .= '| setlocal keywordprg<'
+endif
+
 let b:did_ftplugin = 1