fix(api): nvim_open_tabpage positional "enter"

Problem: nvim_open_tabpage's "enter" argument is optional, which is inconsistent
with nvim_open_win.

Solution: make it a (non-optional) positional argument, like nvim_open_win.

Also change "enter"'s description to be more like nvim_open_win's doc.

runtime/lua/vim/_meta/api_keysets.lua

@@ -438,7 +438,6 @@ error('Cannot require a meta file')
 --- @field _subpriority? integer
 
 --- @class vim.api.keyset.tabpage_config
---- @field enter? boolean
 --- @field after? integer
 
 --- @class vim.api.keyset.user_command