Some get_users_in_context docblocks are incorrect

The /privacy/classes/local/request/core_userlist_provider.php interface method, as well as some providers say:
"Get the list of contexts that contain user information for the specified user."

They need to be corrected to:

"Get the list of users who have data within a context."

Note: Some affected providers include core_group and mod_assignment, but this should be checked once all of the current providers have landed, in case any others have been missed that haven't landed at the time this issue was created.