Remove references to removed $fields parameter

The `get_post()` and `prepare_post()` methods apparently had a
`$fields` parameter at one time. This has been replaced by the
`$context` parameter.

When the `$fields` parameter was removed, some of the inline docs
stayed behind. This removes them.
Add summary of the $context parameter

For the `get_post(s)` methods, I think only the `’view’` context is
used internally, but I’ve referenced `’edit’` there as well.
Merge pull request #326 from JDGrimes/fields-parameter-docs

Remove references to removed $fields parameter