Missing details in API docs

[tleonardi]

Hi,
I've noticed some functions miss the sphinx documentation:
parse_pubmed_paragraph(): the parameter all_paragraph is not documented
table_to_df(): parameters and return value not documented
parse_pubmed_web_tree(): parameters and return value not documented
extract_citations(): parameters and return value not documented
extract_pmc(): parameters and return value not documented

[titipata]

I've fixed this issue in 7d4ae40. I will further update the documentation and point to this issue later on!