DOC: Expand docs for atoms.ratttle to explain seed.

Related to !3333 (merged). Just updated the atoms.rattle docstring to note how random seeds are treated here. (It is different behavior than in MaxwellBoltzmannDistribution, which confused me.)

Note: we should be able to easily remove the constraint on atoms.rattle that the user must provide a seed, presumably with the same lines of code as in the Maxwell-Boltzmann distribution. (I just don't have time to fully test it now, so thought I'd at least update the docs.)

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• [NA] Unit tests have been added for new or changed code.
• [NA] Issue is resolved via "closes #XXXX" if applicable.