(core) A few tiny documentation tweaks.

Summary:
- Improve readability of documentation of CONTAINS.
- Add leading underscore to Record._get_encodable_row_ids() to hide from
  public docs, and avoid interfering with user fields.
- Fix up lint errors

Test Plan: No behavior changes

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3209

sandbox/grist/records.py

@@ -133,6 +133,8 @@ class RecordSet(object):
 
   You can get the number of records in a RecordSet using `len`, e.g. `len($group)`.
   """
+  # Methods should be named with a leading underscore to avoid interfering with access to
+  # user-defined fields.
   def __init__(self, table, row_ids, relation=None, group_by=None, sort_by=None):
     """
     group_by may be a dictionary mapping column names to values that are all the same for the given
@@ -194,7 +196,7 @@ class RecordSet(object):
                                  group_by=self._group_by,
                                  sort_by=self._sort_by)
 
-  def get_encodable_row_ids(self):
+  def _get_encodable_row_ids(self):
     """
     Returns stored rowIds as a simple list or tuple type, even if actually stored as RecordList.
     """