doc: clarify window-id, tab-id, nvim_set_current_x #32528

Problem:
Descriptions are somewhat vague. nvim_set_current_line modifies contents
but nvim_set_current_buf does not, etc.

Solution:
- Make it clear that these functions accept or return a winid/tabid by
  linking to that concept in help.
- Only these few files use the term "handles", so replace them with the
  more conventional terminology.
- Add a new help section for tab-ID. This concept is unique to neovim
  because vim exposes tabnr, but not tab handles. This section is
  modelled after `:h winid`.

src/nvim/api/win_config.c

@@ -214,7 +214,7 @@
 ///
 /// @param[out] err Error details, if any
 ///
-/// @return Window handle, or 0 on error
+/// @return |window-ID|, or 0 on error
 Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Error *err)
   FUNC_API_SINCE(6) FUNC_API_TEXTLOCK_ALLOW_CMDWIN
 {
@@ -387,7 +387,7 @@ static int win_split_flags(WinSplit split, bool toplevel)
 ///
 /// @see |nvim_open_win()|
 ///
-/// @param      window  Window handle, or 0 for current window
+/// @param      window  |window-ID|, or 0 for current window
 /// @param      config  Map defining the window configuration,
 ///                     see |nvim_open_win()|
 /// @param[out] err     Error details, if any
@@ -694,7 +694,7 @@ static void config_put_bordertext(Dict(win_config) *config, WinConfig *fconfig,
 ///
 /// `relative` is empty for normal windows.
 ///
-/// @param      window Window handle, or 0 for current window
+/// @param      window |window-ID|, or 0 for current window
 /// @param[out] err Error details, if any
 /// @return     Map defining the window configuration, see |nvim_open_win()|
 Dict(win_config) nvim_win_get_config(Window window, Arena *arena, Error *err)