[migration] Add clarification for validate() for header only

[danimtb]

closes https://github.com/conan-io/conan/issues/12210

[prince-chrismc]

🤔 Trying to see why my PR is failing the 2.0 pipeline... I hit this issue

https://github.com/conan-io/conan-center-index/blob/318053389c781d02566c2609e9e77a1e205ca00a/recipes/expected-lite/all/conanfile.py#L23-L25

Fails with 2.0 with 'self.settings' access in 'validate()' method is forbidden

I am also having problems with check_min_vs failing for the same reason

string-view-lite/1.6.0: Compatible package ID da39a3ee5e6b4b0d3255bfef95601890afd80709 equal to the default package ID
variant-lite/2.0.0: Compatible package ID da39a3ee5e6b4b0d3255bfef95601890afd80709 equal to the default package ID
ERROR: restinio/0.6.17: Error in validate() method, line 100
        check_min_vs(self, 190)
        ConanException: 'self.settings' access in 'validate()' method is forbidden
 ✘ christopherm@christopherm-mac  ~/conan-center-index/recipes/restinio   update/restinio ±  conan --version
Conan version 2.0.0-beta3

[memsharded]

Yes, that is true, and this is one of the reasons this issue is being problematic, the migration path 1.X->2.0 is not clear yet.

[prince-chrismc]

This needs some attention https://github.com/conan-io/docs/pull/2778/files#diff-633dc3f2388dc1689eaab8f6f51e1435bf9be9a28e73d49d302a583de6360789R185 should be more clear about validate vs package_id

[prince-chrismc]

This PR matches the current recommendations and it's doesn’t have any open questions AFAIK so I will go ahead and merge this tiny snippet