Document water selection keyword

[IAlibay]

PR #4854 added a water selection keyword but was merged without adding any documentation to the selections docs here: https://github.com/MDAnalysis/mdanalysis/blob/develop/package%2Fdoc%2Fsphinx%2Fsource%2Fdocumentation_pages%2Fselections.rst

This needs to be done so that the keyword is appropriately exposed to users.

[RMeli]

@talagayev would you mind please adding such documentation? Thank you!

[talagayev]

@IAlibay, @RMeli yes, I can add the documentation and create a PR for it tomorrow.

Should it be like it is for protein, backbone, nucleic, nucleicbackbone currently in the selections.rst, so that it is an entry under Simple selections or should it also have some additional example of usage?