ENH: Add note about lack of cleaning methods for data indices

[rstoneback]

Description

Add a note or warning to top-level docstring for space weather indices that do not have a cleaning method so that it looks nicely formatted and attention grabbing in documentation. In packages like pysatNASA there is a warning for instruments without cleaning. An index is a more processed dataset than an instrument thus a warning may not be appropriate. Nevertheless, users need guidance somewhere that there is no cleaning.

Potential impact

• Provides clarity for the user

Potential solution(s)

Add a note or warning to top-level docstring for space weather indices that do not have a cleaning method

Alternatives

• Leave as is.
• Add a warning like with pysatNASA. In the log, every load call will have an entry.

Additional context