DOC: Add missing docstrings (#5331)

Follow-up to PR #5325.

Author: ParthUppal523

package/CHANGELOG

@@ -46,6 +46,8 @@ Fixes
    DSSP by porting upstream PyDSSP 0.9.1 fix (Issue #4913)
 
 Enhancements
+ * Added documentation for all keyword in select_atoms() and 
+   selections.rst (Issue #5317, PR #5325)
  * Added HydrogenBondAnalysis benchmark for performance tracking (PR #5309)
  * Added `select=None` in `analysis.rms.RMSD` to perform no selection on
    the input `atomgroup` and `reference` (Issue #5300, PR #5296)

package/MDAnalysis/core/groups.py

@@ -3289,6 +3289,10 @@ def select_atoms(
 
         **Simple selections**
 
+            all
+                selects all atoms in the current group; the resulting
+                :class:AtomGroup is unique and sorted by index. If the group
+                already corresponds to Universe.atoms, it is returned unchanged.
             protein, backbone, nucleic, nucleicbackbone
                 selects all atoms that belong to a standard set of residues;
                 a protein is identfied by a hard-coded set of residue names so