RMSD, RMSF, DensityAnalysis Documentation Update

[nawtrey]

Changes made in this Pull Request:

• Remove "updating_selection" from pmda.density, update to "updating"
• Add list of trajectory requirements from mda.analysis.density to DensityAnalysis docstring
• Add documentation for RMSD: explanation, examples, etc.
• Add Parallel MDAnalysis paper link to rmsf.RMSF and density.DensityAnalysis
• Update 'second_order_moments()' documentation

PR Checklist

• [ ] Tests?
• [ ] Docs?
• [ ] CHANGELOG updated?
• [ ] Issue raised/referenced?