docs: document PreviewWatchingStopped event and update SyncTeX recipes

Problem: the new `PreviewWatchingStopped` event was undocumented, and
the SyncTeX recipes never cleared `synctex_pdf` on watch stop.

Solution: add the event to the events section and update the shared
SyncTeX setup block to clear `synctex_pdf[bufnr]` on stop.

doc/preview.txt

@@ -293,6 +293,9 @@ preview.nvim fires User autocmds with structured data:
 `PreviewCompileFailed`   Compilation failed (non-zero exit).
                         data: `{ bufnr, provider, code, stderr }`
 
+`PreviewWatchingStopped` Watching stopped (toggle off or buffer delete).
+                        data: `{ bufnr, provider }`
+
 Example: >lua
     vim.api.nvim_create_autocmd('User', {
       pattern = 'PreviewCompileSuccess',
@@ -354,6 +357,12 @@ The following configs leverage the below basic setup: ~
         synctex_pdf[args.data.bufnr] = args.data.output
       end,
     })
+    vim.api.nvim_create_autocmd('User', {
+      pattern = 'PreviewWatchingStopped',
+      callback = function(args)
+        synctex_pdf[args.data.bufnr] = nil
+      end,
+    })
 <
 
 The recipes below bind `<leader>s` for forward search. To scroll the PDF