Add information in help doc

Author: mizlan

doc/nnn.txt

@@ -44,7 +44,8 @@ buffer opens running an n³ process in picker mode.
 Once you select (see https://github.com/jarun/nnn/wiki/concepts#selection)
 one or more files and press enter, vim quits the n³ window and opens the
 first selected file and add the remaining files to the arg list/buffer
-list.
+list. If |nnn#session| is enabled, then n³ will remember where you left off
+of when you reopen it.
 
 Pressing enter on a file in n³ will pick any earlier selection, pick
 the file and exit n³.
@@ -146,6 +147,19 @@ g:nnn#action                                                     *g:nnn#action*
                   endfunction
 <
 
+g:nnn#session                                                   *g:nnn#session*
+                  Default: 'none'
+                  How n³ should utilize sessions. Default is 'none' meaning no
+                  persistent sessions. Other options are 'local' for the same
+                  n³ session across a vim session, or 'global' for the same n³
+                  session across everywhere.
+>
+                  " use the same nnn session within a vim session, for
+                  " example, so that consecutive uses of :Np will remember
+                  " where you last left off
+                  let g:nnn#session = 'local'
+<
+
 g:nnn#command                                                   *g:nnn#command*
                   Default: 'nnn'
                   When you want to override the default n³ command and add