runtime(doc): Add environment variable expansion note to options

runtime(doc): Add environment variable expansion note to options

Add "Environment variables are expanded |:set_env|" documentation to
options that have the P_EXPAND flag but were missing this note.

Updated options:
- 'cdpath'
- 'dictionary'
- 'mkspellmem'
- 'packpath'
- 'runtimepath'
- 'spellfile'
- 'spellsuggest'
- 'thesaurus'
- 'ttytype'
- 'undodir'
- 'verbosefile'
- 'viewdir'
- 'viminfofile'

These options support environment variable expansion in their values
(e.g., $HOME, $USER) but the documentation didn't explicitly mention
this capability. This brings their documentation in line with other
options like backupdir, directory, and makeprg that already include
this note.

closes: #18791

Signed-off-by: Alex Plate <AlexPl292@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>