options: set 'scrollback' to -1 by default #9563

Makes the 'scrollback' option more consistent (same default for all buffers) and future-proof.

- Default to -1 for all buffers, but treat it as an implementation detail.
- Document range of 1 - 100_000.
- New terminal buffer by default sets scrollback=10_000 if the global default is -1.
- Existing terminal buffer: On entering terminal-mode or on refresh, if the user explicitly did `:set[local] scbk=-1`, the local value goes to 100_000 (max). (This is undocumented on purpose. Users should work with explicit values in the range of 1-100_000.)

src/nvim/options.lua

@@ -1930,7 +1930,7 @@ return {
       vi_def=true,
       varname='p_scbk',
       redraw={'current_buffer'},
-      defaults={if_true={vi=10000}}
+      defaults={if_true={vi=-1}}
     },
     {
       full_name='scrollbind', abbreviation='scb',