Update members-header.php profile field guide to state bp_member_profile_data()

[hnla]

We are still saying in members-header "to output additional profile fields use bp_profile_field_data() yet that function doesn't appear to want to render an array whereas the newer bp_member_profile_data() does output an array.

Should we not change this comment message and save people getting confused?

Sure, this is a good idea.

(In [6218]) Improved documentation in member-header.php

In member-header.php template, we suggest to theme authors that they can
include arbitrary profile data by calling a template function. Previously
we recommended bp_profile_field_data(). But this function, when called on a
field that contains values stored in an array, does not make the array
presentable. The newer bp_member_profile_data() is better for this purpose.
This changeset updates the documentation accordingly.

Fixes #4319

Props hnla