Document export of package functions in LorentzVectorBase (#33)

* Document export of package functions in LorentzVectorBase
Added notes on exporting package functions and provided an example module.
* Clarify export status of package functions
* spelling from @Copilot

Author: mmikhasenko

docs/src/10-interface.md

@@ -120,6 +120,22 @@ This allows users to choose more descriptive or domain-specific terminology with
 | `azimuthal_angle`      | `phi`              |
 | `pseudorapidity`       | `eta`              |
 
+
+Note, that most of the package functions are not exported. They can always be called with `LorentzVectorBase.` prefix.
+For convenience, these methods can be exported in custom packages that utilize LorentzVectorBase.
+For example, as follows,
+
+```julia
+module Custom4Vector
+
+import LorentzVectorBase: invariant_mass
+export invariant_mass
+
+end
+```
+
+See also [an example](https://github.com/mmikhasenko/FourVectors.jl/blob/main/src/FourVectors.jl#L28-L33) of exporting multiple selected methods in a loop.
+
 ## Coordinate System Tags
 
 The following coordinate systems are supported via tags like `XYZT()`, `PtEtaPhiM()`, etc.: