Add docs and rename import pc -> pacsv

Signed-off-by: Will Beason <willbeason@gmail.com>

tables.py

@@ -33,6 +33,10 @@ class RevisionField(ABC, Generic[T]):
         :param revisions: The set of revisions to compute the field from.
         Revisions are passed in chronological order, so use revisions[-1] to
         access the most recent revision in the set.
+
+        Implementations of extract should handle the case where revisions is
+        either a single revision (collapse-user=FALSE), or a full edit session
+        of contiguous edits by the same user (collapse-user=TRUE).
         """
         pass