feat(defaults): store spellfile in stdpath('data') #33048

Problem:
First rtp directory is unpredictable and not in line with XDG
base spec.

Solution:
Use stdpath('data')/spell as directory if 'spellfile' is not set.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/doc/news.txt

@@ -177,7 +177,8 @@ CHANGED FEATURES                                                 *news-changed*
 
 These existing features changed their behavior.
 
-• todo
+• 'spellfile' location defaults to `stdpath("data").."/spell/"` instead of the
+  first writable directoy in 'runtimepath'.
 
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*

runtime/doc/options.txt

@@ -5755,11 +5755,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	It may also be a comma-separated list of names.  A count before the
 	|zg| and |zw| commands can be used to access each.  This allows using
 	a personal word list file and a project word list file.
-	When a word is added while this option is empty Vim will set it for
-	you: Using the first directory in 'runtimepath' that is writable.  If
-	there is no "spell" directory yet it will be created.  For the file
-	name the first language name that appears in 'spelllang' is used,
-	ignoring the region.
+	When a word is added while this option is empty Nvim will use
+	(and auto-create) `stdpath('data')/spell/`. For the file name the
+	first language name that appears in 'spelllang' is used, ignoring the
+	region.
 	The resulting ".spl" file will be used for spell checking, it does not
 	have to appear in 'spelllang'.
 	Normally one file is used for all regions, but you can add the region

runtime/doc/vim_diff.txt

@@ -79,6 +79,7 @@ Defaults                                            *defaults* *nvim-defaults*
 - 'showcmd' is enabled
 - 'sidescroll' defaults to 1
 - 'smarttab' is enabled
+- 'spellfile' defaults to `stdpath("data").."/spell/"`
 - 'startofline' is disabled
 - 'switchbuf' defaults to "uselast"
 - 'tabpagemax' defaults to 50

runtime/lua/vim/_meta/options.lua

@@ -6123,11 +6123,10 @@ vim.bo.spc = vim.bo.spellcapcheck
 --- It may also be a comma-separated list of names.  A count before the
 --- `zg` and `zw` commands can be used to access each.  This allows using
 --- a personal word list file and a project word list file.
---- When a word is added while this option is empty Vim will set it for
---- you: Using the first directory in 'runtimepath' that is writable.  If
---- there is no "spell" directory yet it will be created.  For the file
---- name the first language name that appears in 'spelllang' is used,
---- ignoring the region.
+--- When a word is added while this option is empty Nvim will use
+--- (and auto-create) `stdpath('data')/spell/`. For the file name the
+--- first language name that appears in 'spelllang' is used, ignoring the
+--- region.
 --- The resulting ".spl" file will be used for spell checking, it does not
 --- have to appear in 'spelllang'.
 --- Normally one file is used for all regions, but you can add the region