Clarify Validate method functionality in doc comments

[peterstace]

Description

As per title.

Check List

Have you:

• Added unit tests? N/A

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) N/A

Related Issue

• Part of https://github.com/peterstace/simplefeatures/issues/504

[peterstace]

Thanks for reviewing @albertteoh , I really appreciate it 😁