Clarify country search logic

[IsabelBirds]

Hi @notshi @xriss
As discussed I'm adding some text to the docs to clarify what people should expect when they filter by a country.
I can't tag you in the docs issue - could you comment there please? IATI/iati-d-portal-docs#25
Thanks!

[IsabelBirds]

Hiya, just following up on this! Thanks both :) cc @stevieflow