docs: notify undefined result

[ivokub]

Description

Notifies users that the scalar multiplication result is undefined in case the input point is not in the prime order subgroup.

Resolves #1484.

cc @yelhousni - right now I only added comments. But dunno if we would like to add an option which would enable using "naive scalar multiplication" for all points? I'm not sure if it would be necessary anywhere, but maybe could be useful?

Type of change

• This change requires a documentation update

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[yelhousni]

LGTM! adding a comment is enough IMO for now, as anyway the GLV endomorphism used is only defined on the prime subgroups. For the generic case we can use scalarMulJoye. Maybe adding a comment there too would make things clearer.