clarify docs

docs/src/elements.md

@@ -98,6 +98,27 @@ add_element!
 remove_custom_elements!
 ```
 
+## Additional property-retrieving functions
+
+The following functions are supported as part of the API, as a intending to interface
+with `AtomsBase`. Nevertheless, currently these functions do not overload the exported
+ones from `AtomsBase`, because that package is in a unstable state.
+
+| Function   |  Example              |  Output |
+|:-----------|:----------------------|:-------:|
+|`atomic_number(::PDBTools.Atom)` | `atomic_number(Atom(name="NE2"))` |  `7` |
+|`atomic_symbol(::PDBTools.Atom)` |  `atomic_symbol(Atom(name="NE2"))` |  `:N` |
+|`atomic_mass(::PDBTools.Atom)`   |  `atomic_mass(Atom(name="NE2"))` |  `14.0067` |
+|`position(::PDBTools.Atom)`      |  `position(Atom(name="NE2"))` |  `SVector{3,Float64}(0,0,0)` |
+
+
+```@docs
+atomic_number
+atomic_symbol
+atomic_mass
+position
+```
+