Update docs relating to the --export option for sbatch, srun

Explain gotchas with --export when an implicit --get-user-env is
triggered.
Correct example in the docs; `--export=EDITOR,ALL` does not make any
sense.
Explain how env vars are propagated with srun and sbatch when `ALL` is
specified; document their differences.
Explain that srun --export is implemented in the Launch plugin.
Fix minor grammatical errors.
Remove --export reference in salloc.

Bug 6042