[Documentation] Clarify credit field usage in APOD API

[StellarStoic]

Description:

This pull request aims to address a gap in the documentation regarding the credit field in the APOD API responses.

During the usage of the APOD API, it was noticed that sometimes the copyright field is not present and a field named credit appears to be in its place. This was not documented in the current API guide, leading to potential misunderstandings and errors when developers attempt to access the copyright or credit field.

Changes Proposed:

Add documentation to the APOD API guide clarifying the usage and conditions of the credit field.
Provide examples in the API guide of responses that might contain the credit field instead of the copyright field.
Update any existing examples in the documentation that refer to the copyright field, noting that credit may sometimes be present instead.
By providing this additional clarity around the credit field in the APOD API documentation, we can help developers avoid unexpected errors and better handle the variability in the API responses.

[Dan-Banfield]

Agreed. Experienced this issue myself.