Problem:
man/ contains source files for manpage generation, it doesn't need to
live at the project root.

Solution:
Move it to src/man/.

See: https://gitlab.freedesktop.org/xdg/xdg-specs/-/commit/4f2884e16db35f2962d9b64312917c81be5cb54b

- Move session persistent data to $XDG_STATE_HOME Change 'directory',
  'backupdir', 'undodir', 'viewdir' and 'shadafile' default location to
  $XDG_STATE_HOME/nvim.
- Move logs to $XDG_STATE_HOME, too.
- Add stdpath('log') support.

Fixes: #14805

The phrase referred specifically to `$VIMINIT` and `$EXRC`, which
are parsed (and available with, e.g., `echo $VIMINIT` if set) but
of course not loaded since _any_ initialization is skipped. Hence
this is redundant and can be misleading.

Update runtime files
https://github.com/vim/vim/commit/2346a6378483c9871016f9fc821ec5cbea638f13

omit doc/vim.man
omit doc/vim9.txt
omit doc/vimdiff.txt
omit doc/vimdiff.man

skip doc/eval.txt (missing patch 8.2.2324)
skip doc/help.txt (missing patch 8.2.2344)

while there is some controversy, stdpath('cache') looks like  a better fit for logs than stdpath('data'): you can remove logs without preventing nvim to work which fits the XDG_CACHE_HOME definition of `user specific non-essential data files`.

As per #11996, restricted mode (-Z) has been removed. Nonetheless in #11996 the man page was not updated. Hence this PR.