Manual: Document `TMPDIR` env variable
Manual: Document `TMPDIR` env variable

Closes #873

restic/restic,kurin/restic,middelink/restic,ar-jan/restic,byo/restic,jayme-github/restic,stuertz/restic,fawick/restic,fawick/restic,middelink/restic,kurin/restic,byo/restic,stuertz/restic,ar-jan/restic,restic/restic,jayme-github/restic