docspack): mention re-install after source's default branch change #37560

Problem: `vim.pack.update()` doesn't update source's default branch if
  it has changed on the remote. It can be done by executing
  `git remote set-head origin --auto` for every plugin on every update,
  but it feels like too much extra work (which requires Internet
  connection) for a very rare use case.

  This matters since `version = nil` will keep pointing to previous
  default branch even after `vim.pack.update()`.

Solution: Document that in order for `version = nil` to point to the
  source's new default branch, perform clean re-install.

runtime/lua/vim/pack.lua

@@ -1210,6 +1210,9 @@ end
 ---
 --- Notes:
 --- - Every actual update is logged in "nvim-pack.log" file inside "log" |stdpath()|.
+--- - It doesn't update source's default branch if it has changed (like from `master` to `main`).
+---   To have `version = nil` point to a new default branch, re-install the plugin
+---   (|vim.pack.del()| + |vim.pack.add()|).
 ---
 --- @param names? string[] List of plugin names to update. Must be managed
 --- by |vim.pack|, not necessarily already added to current session.