Addition of water selection documentation (#4881)

* Update selections.rst

addition of water in simple selection with its description
added mention of all available resnames for water that are recognized

---------

Co-authored-by: Irfan Alibay <[email protected]>

package/doc/sphinx/source/documentation_pages/selections.rst

@@ -114,6 +114,15 @@ protein, backbone, nucleic, nucleicbackbone
     is identfied by a hard-coded set of residue names so it  may not
     work for esoteric residues.
 
+water
+    selects all atoms that belong to a set of water residues; water
+    is defined with a set of common water abbreviations present in
+    topology files and may not work with certain water residue names.
+    Currently the following water resnames are supported:
+    3 letter resnames: ``H2O``, ``HOH``, ``OH2``, ``HHO``, ``OHH``, ``TIP``,
+    ``T3P``, ``T4P``, ``T5P``, ``SOL``, ``WAT``.
+    4 letter resnames: ``TIP2``, ``TIP3``, ``TIP4``.
+
 segid *seg-name*
     select by segid (as given in the topology), e.g. ``segid 4AKE`` or
     ``segid DMPC``