docs: add :DiffviewToggle docs (sindrets#517)

Author: cosmicbuffalo

doc/diffview.txt

@@ -18,12 +18,12 @@ for any git rev.
 
 USAGE                                           *diffview-usage*
 
-Quick-start: `:DiffviewOpen` to open a Diffview that compares against the
+Quick-start: `:DiffviewOpen` or `:DiffviewToggle` to open a Diffview that compares against the
 index.
 
 You can have multiple Diffviews open, tracking different git revs. Each Diffview
-opens in its own tabpage. To close and dispose of a Diffview, call either
-`:DiffviewClose` or `:tabclose` while a Diffview is the current tabpage.
+opens in its own tabpage. To close and dispose of a Diffview, call
+`:DiffviewClose`, `:DiffviewToggle` or `:tabclose` while a Diffview is the current tabpage.
 Diffviews are automatically updated:
 
         • Every time you enter a Diffview
@@ -370,6 +370,13 @@ COMMANDS                                        *diffview-commands*
                                                 *:DiffviewClose*
 :DiffviewClose          Close the active Diffview.
 
+                                                *:DiffviewToggle*
+:DiffviewToggle [git-rev] [options] [ -- {paths...}]
+
+       Alias for `:DiffviewOpen` when the current tabpage is not a Diffview,
+       otherwise acts as an alias for `:DiffviewClose`. Accepts the same
+       arguments as `:DiffviewOpen`.
+
                                                 *:DiffviewToggleFiles*
 :DiffviewToggleFiles    Toggles the file panel.
 

lua/diffview/init.lua

@@ -133,7 +133,6 @@ function M.open(args)
   end
 end
 
-
 ---@param range? { [1]: integer, [2]: integer }
 ---@param args string[]
 function M.file_history(range, args)