Add new slash command to the docs.

app/services/slash_commands/interpret_service.rb

@@ -257,8 +257,8 @@ module SlashCommands
       @updates[:time_estimate] = ChronicDuration.parse(raw_duration, default_unit: 'hours')
     end
 
-    desc 'Enter current spent time'
-    params '<1h 30m>'
+    desc 'Add or substract spent time'
+    params '<1h 30m | -1h 30m>'
     condition do
       current_user.can?(:"admin_#{issuable.to_ability_name}", issuable)
     end

doc/user/project/slash_commands.md

@@ -29,3 +29,5 @@ do.
 | <code>/due &lt;in 2 days &#124; this Friday &#124; December 31st&gt;</code> | Set due date |
 | `/remove_due_date`         | Remove due date |
 | `/wip`                     | Toggle the Work In Progress status |
+| <code>/estimate &lt;1w 3d 2h 14m&gt;</code> | Set time estimate |
+| <code>/spend &lt;1h 30m &#124; -1h 5m&gt;</code> | Add or substract spent time |