Update gene-page-literature.md

src/docs/documentation/gene-page-literature.md

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 ## Gene page: Literature
 
 The Literature section of a gene page lists the papers that have curated
-data for the gene or its product.
+data for the gene or its product, and papers which mention the gene in keywords.
 
 At the top of the section, there is a link to search PubMed for any
 papers on the gene. This may retrieve papers not on the gene page