Clarify the documentation of the primary_key option for associations

Previously the documentation stated that `primary_key` should be the name of
a *method* that returns the primary key used for the association. This is
incorrect. This changes the documentation to say that the value must be the
name of a column.
Merge pull request #17012 from petedmarsh/clarify-association-primary-key-option-documentation

Clarify the documentation of the primary_key option for associations