clarify SLURM job handler docs

[wxtim]

Clarify the formats permitted for directives to SLURM.

Record investigations:

# part of a flow.cylc file
[[[directives]]]
  -H               # Fails Cylc validation
  -H=              # Works
  --hold           # Fails Cylc validation
  --hold=          # Holds your SLURM job as you'd hope
  -b now+60        # Fails Cylc validation
  -b=now+60        # Sbatch fails with invalid options
  -b now+60=       # Sbatch ... just no
  --begin=now+60   # Works

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• This is a documentation change only: I have built the documentation locally.