(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/functions/math.py

@@ -612,6 +612,8 @@ def ROUND(value, places=0):
   2.0
   >>> ROUND(2.5)
   3.0
+  >>> ROUND(-2.5)
+  -3.0
   >>> ROUND(2.15, 1)
   2.2
   >>> ROUND(-1.475, 2)