(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/lookup.py

@@ -154,7 +154,9 @@ def VLOOKUP(table, **field_value_pairs):
 class _Contains(namedtuple("_Contains", "value")):
   """
   Use this marker with [UserTable.lookupRecords](#lookuprecords) to find records
-  where a field of a list type (such as `Choice List` or `Reference List`) contains the given value, e.g:
+  where a field of a list type (such as `Choice List` or `Reference List`) contains the given value.
+
+  For example:
 
       MoviesTable.lookupRecords(genre=CONTAINS("Drama"))
 

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)