Update configure maf docstring (#578)

* add to docstring about maf germline filtering

* lint

Author: rxu17

genie/database_to_staging.py

@@ -273,6 +273,15 @@ def get_whitelist_variants_idx(mafdf):
 def configure_maf(mafdf, remove_variants, flagged_variants):
     """Configures each maf dataframe, does germline filtering
 
+    Germline filtering for MAF files uses the gnomAD columns that refer to the
+    allele frequencies (AF) of variants in different population groups
+    from the gnomAD (Genome Aggregation Database). This filter will filter out
+    variants with a maximum AF > 0.05% across all populations which are typically
+    common germline variants.
+
+    Germline filtering for MAF files occurs during release instead of during processing
+    because the MAF file gets re-annotated during processing via genome nexus annotation.
+
     Args:
         mafdf: Maf dataframe
         remove_variants: Variants to remove