Replace docstring "Placeholder" text

[mfisher87]

Recently, we added placeholder subject lines to docstrings which lacked subject lines but contained other data, like parameter annotations or return annotation. We needed to do this because:

• the code formatter didn't understand this situation and would try to fix it by bringing the "Returns:" line to the subject, and
• this would cause the docs to no longer correctly render the return type

We need to replace "Placeholder" with a real subject line for each of those functions/classes.