docs(spell): can't add word to personal dict

Adding `Cannot add [word] to any active dictionary` error to the
troubleshooting section.

I have had this problem multiple times now when I break my emacs and
wipe the `~/.config/emacs` directory and I can never remember what file
to create to get my spell check working properly.

Even though this is a bug and might be fixed in the future, I feel like
having it documented can be helpful in the short term and in the future.

Ref: #6246

modules/checkers/spell/README.org

@@ -208,6 +208,24 @@ installed, it will generate an empty word list, causing it to highlight all
 words as incorrect. Delete its cache files in =$EMACSDIR/.local/etc/spell-fu/=
 to fix this.
 
+** Cannot add [word] to any active dictionary
+*** Aspell
+If ~M-x +spell/add-word~ results in the above error this mean that the
+personal dictionary file was not created for some reason. This can be
+fixed by creating the required file manually.
+#+begin_example shell
+mkdir -p ~/.emacs.d/.local/etc/ispell
+echo personal_ws-1.1 en 0 > ~/.emacs.d/.local/etc/ispell/.pws
+#+end_example
+
+Where ~personal_ws-1.1 en 0~ is the required header format for the personal
+dictionary file. ~en~ is the language you're writing in and have a dict
+installed and ~0~ is the number of added words in the dictionary. If you are
+planning of updating the file with the list of words, update the number
+accordingly.
+
+After the file is created, restart emacs and adding words should work.
+
 * Frequently asked questions
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